UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 6)

American Greetings Corporation

(Name of Issuer)

Class B Common Shares, Par Value $1.00

(Title of Class of Securities)

026375-20-4

(CUSIP Number)

Zev Weiss

Jeffrey Weiss

One American Road

Cleveland, Ohio 44144

(216) 252-7300

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

Copies to:

James P. Dougherty

Jones Day

901 Lakeside Avenue

Cleveland, Ohio 44114

(216) 586-3939

July 3, 2013

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Explanatory Note

This Amendment No. 6 to Schedule 13D (this “Amendment”) amends the Schedule 13D filed by the Reporting Persons on September 26, 2012, as amended by Amendment No. 1 to Schedule 13D dated November 5, 2012, Amendment No. 2 to Schedule 13D dated December 26, 2012, Amendment No. 3 to Schedule 13D dated January 17, 2013, Amendment No. 4 to Schedule 13D dated April 1, 2013 and Amendment No. 5 to Schedule 13D dated July 1, 2013 (the “Schedule 13D”). Capitalized terms used but not otherwise defined in this Amendment have the meanings ascribed to such terms in the Schedule 13D.

Item 3. Source and Amount of Funds or Other Consideration

Item 3 is hereby amended and restated in its entirety as follows:

The aggregate value of the transaction contemplated by the Merger Agreement and as further described in Item 4 below (the “Transaction”), including debt incurred, refinanced or to remain outstanding in connection with the Transaction, is $879.1 million.

The Transaction will be financed through a combination of (i) cash funded by a $240.0 million non-voting preferred stock investment committed by Koch AG Investment, LLC, a Delaware limited liability company (“Koch”) pursuant to a Series A Preferred Stock Purchase Agreement, dated as of March 29, 2013 (the “Stock Purchase Agreement”), among Century Intermediate Holding Company, a Delaware corporation (“Parent”), Koch, and, solely for purposes of Section 12.9 thereof, Zev Weiss, Morry Weiss, Jeffrey Weiss and Koch Industries, Inc., and (ii) $600 million in committed debt financing, consisting of a $350 million term loan and a $250 million revolving credit facility pursuant to a Commitment Letter, dated March 29, 2013 (the “Commitment Letter”), among Parent, Bank of America, N.A., Deutsche Bank AG New York Branch, Key Bank National Association, Macquarie Capital USA, Inc. and PNC Bank National Association (the “Commitment Parties”), as amended by the Amendment to Commitment Letter, dated July 3, 2013 (the “Commitment Letter Amendment”), among Parent and the Commitment Parties. The Stock Purchase Agreement, the Commitment Letter and the Commitment Letter Amendment are filed as Exhibits 6, 7 and 14 hereto, respectively, and are incorporated by reference into this Item 3. The foregoing descriptions of the Stock Purchase Agreement, the Commitment Letter and the Commitment Letter Amendment do not purport to be complete, and are qualified in their entirety by reference to the full text of those documents.

In addition, the Reporting Persons (excluding the Irving Stone Foundation) entered into a Rollover and Contribution Agreement, dated March 29, 2013 (the “Rollover Agreement”), among the Reporting Persons (other than the Irving Stone Foundation) (the “Family Shareholders”), Parent and Three-Twenty-Three Family Holdings, LLC, a Delaware limited liability company (“Family LLC”). Pursuant to the Rollover Agreement, the Family Shareholders will, subject to the terms and conditions contained therein and immediately prior to the effective time of the Merger (as defined below), contribute approximately 2.33 million Class A Shares and, Class B Shares, consisting of all of the Class A Shares and Class B Shares owned by the Family Shareholders (the “Rolled Shares”) to Family LLC in exchange for all of the equity interests in Family LLC. Family LLC will, in turn, contribute the Rolled Shares to Parent in exchange for all of the common equity interests in Parent. The Issuer is not a party to the Rollover Agreement. The Rollover Agreement is filed as Exhibit 8 hereto and is incorporated by reference into this Item 3. The foregoing description of the Rollover Agreement does not purport to be complete, and is qualified in its entirety by reference to the full text thereof.

Item 4. Purpose of Transaction

Item 4 is hereby supplemented with the following information:

On July 3, 2013, the Issuer published a press release (the “Amendment Press Release”) announcing that it had entered into Amendment No. 1 to Agreement and Plan of Merger, dated July 3, 2013 (the “Merger Agreement Amendment”), among Parent, Merger Sub and the Issuer, amending the Merger Agreement and providing for, among other things, the consideration to be received in respect of the shares of the Company (other than shares owned by the Issuer, Parent (which will include at the effective time all the shares held by the Family Shareholders) and those holders who have properly exercised dissenters’ rights under Ohio law) to be increased from $18.20 to $19.00 (in both cases in cash, without interest and subject to any withholding taxes). The Amendment Press Release and the Merger Agreement Amendment are filed as Exhibits 15 and 16 hereto, respectively, and incorporated by reference into this Item 4. The foregoing descriptions of Amendment Press Release and the Merger Agreement Amendment do not purport to be complete, and are qualified in their entirety by reference to the full text of those documents.

In connection with the execution of the Merger Agreement Amendment, the Reporting Persons entered into Amendment No. 1 to Guaranty and Voting Agreement, dated July 3, 2013 (the “Guaranty and Voting Agreement Amendment”), among the Reporting Persons and the Issuer, amending the Guaranty and Voting Agreement and providing that, if so requested by a majority of the board of directors of the Issuer (excluding the Family Shareholders who are members of the board) at the meeting of the shareholders of the Issuer with respect to the Transaction or any adjournment thereof, the Reporting Persons will vote their shares of the Issuer in favor of any adjournment, postponement or recess. The Guaranty and Voting Agreement Amendment is filed as Exhibit 17 hereto and is incorporated by reference into this Item 4. The foregoing description of the Guaranty and Voting Agreement Amendment does not purport to be complete, and is qualified in its entirety by reference thereto.

Item 6. Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer.

Item 6 is hereby supplemented with the following information:

See the discussion in Item 4 regarding the Merger Agreement Amendment and the Guaranty and Voting Agreement Amendment. Such descriptions of the Merger Agreement Amendment and the Guaranty and Voting Agreement Amendment do not purport to be complete, and are qualified in their entirety by reference to the full text of the Merger Agreement Amendment and the Guaranty and Voting Agreement Amendment, which are filed as Exhibits 16 and 17 hereto, respectively, and incorporated by reference into this Item 6.

Item 7. Material to be Filed as Exhibits.

Item 7 is hereby amended by inserting at the end thereof the following:

SIGNATURES

After reasonable inquiry and to the best of our knowledge and belief, we certify that the information set forth in this statement is true, complete and correct.

Date: July 3, 2013

Exhibit 14

CONFIDENTIAL

July 3, 2013

Century Intermediate Holding Company

1 American Road

Cleveland, Ohio 44144

Attention: Zev Weiss

Project Century

Amendment to Commitment Letter

Ladies and Gentlemen:

Reference is made to that certain Commitment Letter dated as of March 29, 2013 (the “Commitment Letter”) among you, Bank of America, N.A. (“Bank of America”) and Merrill Lynch, Pierce, Fenner & Smith Incorporated (“MLPFS” and, together with Bank of America, “BAML”), Deutsche Bank AG New York Branch (“DBNY”) and Deutsche Bank Securities Inc. (“DBSI” and, together with DBNY, “Deutsche Bank”), PNC Bank, National Association (“PNC Bank”) and PNC Capital Markets LLC (“PNC Capital” and, together with PNC Bank, “PNC”), KeyBank National Association (“KeyBank”), and Macquarie Capital (USA), Inc. (“Macquarie Capital”) and MIHI LLC (“MIHI” and together with Macquarie Capital, “Macquarie”; BAML, Deutsche Bank, PNC, KeyBank and Macquarie are hereinafter referred to as the “Commitment Parties”, “we” or “us”) regarding the arrangement, underwriting and syndication of senior secured credit facilities (the “Senior Credit Facilities”) for the purpose of financing in part the Acquisition (as defined in the Commitment Letter). Unless otherwise defined herein, capitalized terms shall have the same meanings as specified therefor in the Commitment Letter.

Each of the undersigned hereby agrees that, from and after the date hereof, the Commitment Letter shall be amended as follows:

1. The reference to “$584.5 million” in the first paragraph of the Commitment Letter is replaced with a reference to “$611.9 million”.

2. Clause (i) under the sub-heading “Revolving Credit Facilities” under the heading “Senior Credit Facilities” in Annex I to the Commitment Letter is amended and restated in its entirety to read as follows:

“(i) on the Closing Date, solely to fund (x) additional original issue discount and/or upfront fees that may be imposed pursuant to the exercise of the market flex provisions in the Fee Letter, and (y) up to $25.0 million to finance in part the Acquisition and”

3. The first sentence of clause (i) in Annex II to the Commitment Letter is amended and restated in its entirety to read as follows:

“The Acquisition shall be consummated pursuant to the Agreement and Plan of Merger provided to the Lead Arrangers by email at approximately 2:43 p.m., prevailing New York time, on March 29, 2013 (as amended pursuant to that certain Amendment No. 1 to Agreement and Plan of Merger dated as of July 3, 2013 and subject to any other modifications, consents or waivers thereto that comply with the provisions of this clause (i), and together with the exhibits and schedules thereto, the “Acquisition Agreement”) concurrently with the closing of the Senior Credit Facilities.”

This Amendment to Commitment Letter is subject to the confidentiality provisions set forth in Section 6 of the Commitment Letter. This Amendment to Commitment Letter may be executed in any number of counterparts, each of which when executed will be an original, and all of which, when taken together, will constitute one agreement. Delivery of an executed counterpart of a signature page of this Amendment to Commitment Letter by facsimile transmission or electronic transmission (in pdf format) will be effective as delivery of a manually executed counterpart hereof.

This Amendment to Commitment Letter shall be governed by, and construed in accordance with, the laws of the State of New York.

[Signature Pages Follow]

We are pleased to have the opportunity to work with you in connection with this important financing.

Very truly yours,

BANK OF AMERICA, N.A.

MERRILL LYNCH, PIERCE, FENNER & SMITH INCORPORATED

Signature Page to Amendment to Commitment Letter – Project Century

DEUTSCHE BANK AG NEW YORK BRANCH

DEUTSCHE BANK SECURITIES INC.

Signature Page to Amendment to Commitment Letter – Project Century

Signature Page to Amendment to Commitment Letter – Project Century

Signature Page to Amendment to Commitment Letter – Project Century

Signature Page to Amendment to Commitment Letter – Project Century

Accepted and agreed to as of the date first written above:

Signature Page to Amendment to Commitment Letter – Project Century

Exhibit 15

American Greetings Announces Amendment to Merger Agreement

CLEVELAND, July 3, 2013 – American Greetings Corporation (NYSE: AM) announced today that the merger agreement under which the Weiss Family will acquire the Company has been amended to increase the purchase price per share from $18.20 to $19.00 in cash.

The $19.00 per share price represents a premium of 32.5% over the trading price for Class A shares on September 25th, the date on which the Weiss Family initially proposed to acquire the Company, and a premium of 4.4% to the $18.20 per share price.

Completion of the transaction is subject to certain closing conditions, including a condition that the holders of a majority of all outstanding shares not held by the Weiss Family or by any director or executive officer of the Company or any of its subsidiaries approve the merger.

The Weiss Family includes the Company’s Chairman, Morry Weiss, Director and Chief Executive Officer, Zev Weiss, and Director, President and Chief Operating Officer, Jeffrey Weiss, and related persons and entities.

Chief Executive Officer Zev Weiss said, “Our family remains committed to the transaction and to our belief that American Greetings will be able to best navigate the coming period of reinvestment and transformation as a privately held company.”

Further information regarding the amendment to the definitive merger agreement will be contained in a Current Report on Form 8-K which will be filed promptly with the SEC.

Peter J. Solomon & Company is acting as financial advisor, and Sullivan & Cromwell LLP is acting as legal advisor to the Special Committee of American Greetings’ Board of Directors. Baker & Hostetler LLP is acting as legal advisor to American Greetings. KeyBanc Capital Markets and MacQuarie Capital are acting as financial advisors to the Weiss Family, and Jones Day is acting as legal advisor to the Weiss Family.

About American Greetings Corporation

For more than 100 years, American Greetings Corporation (NYSE: AM) has been a creator and manufacturer of innovative social expression products that assist consumers in enhancing their relationships to create happiness, laughter and love. The Company’s major greeting card lines are American Greetings, Carlton Cards, Gibson, Recycled Paper Greetings and Papyrus, and other paper product offerings include DesignWare party goods and American Greetings and Plus Mark gift-packaging and boxed cards. American Greetings also has one of the largest collections of greetings on the Web, including greeting cards available at Cardstore.com and electronic greeting

cards available at AmericanGreetings.com. In addition to its product lines, American Greetings creates and licenses popular character brands through the American Greetings Properties group. Headquartered in Cleveland, Ohio, American Greetings generates annual revenue of approximately $1.9 billion, and its products can be found in retail outlets worldwide. For more information on the Company, visit http://corporate.americangreetings.com.

Forward Looking Statement Disclosure

Statements about the expected timing, completion and effects of the proposed transaction and all other statements in this release, other than historical facts, constitute forward-looking statements within the meaning of the safe harbor provisions of the Private Securities Litigation Reform Act of 1995. Readers are cautioned not to place undue reliance on these forward-looking statements and any such forward-looking statements are qualified in their entirety by reference to the following cautionary statements. All forward-looking statements speak only as of the date hereof and are based on current expectations and involve a number of assumptions, risks and uncertainties that could cause the actual results to differ materially from such forward-looking statements. The Company may not be able to complete the proposed transaction on the terms described above or other acceptable terms or at all because of a number of factors, including the failure to obtain shareholder approval or the failure to satisfy other closing conditions. Factors that may affect the business or financial results of the Company are described in the risk factors included in the Company’s filings with the Securities and Exchange Commission (the “SEC”), including the Company’s 2013 Annual Report on Form 10-K. The Company expressly disclaims a duty to provide updates to forward-looking statements, whether as a result of new information, future events or other occurrences.

Additional Information and Where to Find It

In connection with the proposed merger transaction, the Company has filed with the SEC a preliminary proxy statement and other relevant documents, and will furnish a definitive version of the proxy statement and other relevant documents to the Company’s shareholders. This press release does not constitute a solicitation of any vote or approval. Shareholders are urged to read the definitive proxy statement when they receive it and any other documents filed with the SEC in connection with the proposed merger or incorporated by reference in the definitive proxy statement, because they will contain important information about the proposed merger.

Investors will be able to obtain a free copy of documents filed with the SEC at the SEC’s website at http://www.sec.gov. In addition, investors may obtain a free copy of the Company’s filings with the SEC from the Company’s website investors section of the Company’s Web site at http://investors.americangreetings.com or by directing a request to the Company’s Corporate Secretary at our World Headquarters address at One American Road, Cleveland, Ohio 44144-2398, or via email to investor.relations@amgreetings.com

Participants in the Solicitation

The Company and its directors, executive officers and certain other members of management and employees of the Company may be deemed “participants” in the solicitation of proxies from shareholders of the Company in favor of the proposed merger. Information regarding the persons who may, under the rules of the SEC, be considered participants in the solicitation of the shareholders of the Company in connection with the proposed merger will be set forth in the definitive proxy statement and the other relevant documents filed with the SEC. You can find information about the Company’s executive officers and directors in its Annual Report on Form 10-K for the fiscal year ended February 28, 2013.

CONTACT:

Gregory M. Steinberg

Treasurer and Executive Director of Investor Relations

American Greetings Corporation

216-252-4864

investor.relations@amgreetings.com

Exhibit 16

EXECUTION VERSION

AMENDMENT NO. 1 TO AGREEMENT AND PLAN OF MERGER

Amendment No. 1 to Agreement and Plan of Merger (this “Amendment”), dated as of July 3, 2013, among Century Intermediate Holding Company, a Delaware corporation (“Parent”), Century Merger Company, an Ohio corporation and a wholly owned subsidiary of Parent (“Merger Sub”), and American Greetings Corporation, an Ohio corporation (the “Company” and, together with Parent and Merger Sub, the “Parties”).

RECITALS

A. The Parties are party to an Agreement and Plan of Merger, dated March 29, 2013 (the “Merger Agreement”).

B. The Parties desire to amend the Merger Agreement pursuant to and in accordance with Section 8.9 of the Merger Agreement, as set forth herein.

NOW, THEREFORE, the Parties agree as follows:

1. Definitions. For the purposes of this Amendment, terms used with initial capital letters and not otherwise defined herein will have the meanings assigned to them in the Merger Agreement. References to articles and sections in this Amendment refer to such articles and sections of the Merger Agreement.

2. Amendment. The Merger Agreement is hereby amended as follows:

1

3. Binding Effect. Except to the extent expressly provided herein, the Merger Agreement will remain in full force and effect in accordance with its terms.

4. Representations and Warranties of the Company. The Company represents and warrants to Parent and Merger Sub as follows: The Company has the requisite corporate power and authority to enter into this Amendment and, subject to receipt of the Company Shareholder Approval and the Minority Shareholder Approval, to consummate the Transactions. The execution and delivery of this Amendment and the consummation of the Transactions have been duly and validly authorized by the Company Board, acting upon the unanimous recommendation of the Special Committee, and, except for the Company Shareholder Approval and the Minority Shareholder Approval, no other corporate proceedings on the part of the Company are necessary to authorize the consummation of the Transactions. The Company Board (acting upon the unanimous recommendation of the Special Committee) has unanimously (by all members of the Company Board participating and with the Officer Shareholders abstaining) determined that the Merger Agreement as amended by this Amendment is in the best interests of the Company and its shareholders (other than the Family Shareholders, Parent and Merger Sub) and declared it advisable to enter into this Amendment, has approved this Amendment, and has resolved to recommend that the Company’s shareholders adopt the Merger Agreement as amended by this Amendment. This Amendment has been duly and validly executed and delivered by the Company and, assuming this Amendment constitutes the valid and binding agreement of Parent and Merger Sub, constitutes the valid and binding agreement of the Company, enforceable against the Company in accordance with its terms, subject to the Bankruptcy and Equity Exception.

2

5. Representations and Warranties of Parent and Merger Sub. Parent and Merger Sub represent and warrant to the Company as follows: Each of Parent and Merger Sub has the requisite corporate power and authority to enter into this Amendment and, in the case of Family LLC, Parent and Merger Sub, to consummate the transactions contemplated by the Merger Agreement as amended by this Amendment. The execution and delivery of this Amendment, in the case of Parent and Merger Sub, and the consummation of the transactions contemplated by the Merger Agreement as amended by this Amendment, in the case of Family LLC, Parent and Merger Sub, have been duly and validly authorized by the Boards of Directors of Parent and Merger Sub, and the members of Family LLC having the authority to authorize such transactions, as applicable, and, except for the adoption of the Merger Agreement as amended by this Amendment by Parent as sole shareholder of Merger Sub (which will occur immediately after the execution and delivery of this Amendment), no other corporate or equivalent proceedings on the part of Family LLC, Parent or Merger Sub are necessary to authorize the consummation of the transactions contemplated thereby. This Amendment has been duly and validly executed and delivered by Parent and Merger Sub and, assuming this Amendment constitutes the valid and binding agreement of the Company, this Agreement constitutes the valid and binding agreement of Parent and Merger Sub, enforceable against each of Parent and Merger Sub in accordance with its terms, subject to the Bankruptcy and Equity Exception.

6. Miscellaneous. Article 8 of the Merger Agreement is incorporated herein by reference and, to the extent applicable, will govern the terms of this Amendment.

[Signature Page Follows]

3

IN WITNESS WHEREOF, the Parties have caused this Amendment to be duly executed and delivered as of the date first above written.

SIGNATURE PAGE TO AMENDMENT NO. 1 TO AGREEMENT AND PLAN OF MERGER

Exhibit 17

EXECUTION VERSION

AMENDMENT NO. 1 TO GUARANTY AND VOTING AGREEMENT

Amendment No. 1 to Guaranty and Voting Agreement (this “Amendment”), dated as of July 3, 2013, among American Greetings Corporation, an Ohio corporation (the “Guaranteed Party”) and each of the shareholders of the Guaranteed Party listed on the signature pages hereto (each a “Family Shareholder”, and, together with the Guaranteed Party, the “Parties”).

RECITALS

A. The Parties are party to a Guaranty and Voting Agreement, dated March 29, 2013 (the “Guaranty and Voting Agreement”).

B. The Parties desire to amend the Guaranty and Voting Agreement pursuant to and in accordance with Section 5 of the Guaranty and Voting Agreement, as set forth herein.

NOW, THEREFORE, the Parties agree as follows:

1. Definitions. For the purposes of this Amendment, terms used with initial capital letters and not otherwise defined herein will have the meanings assigned to them in the Guaranty and Voting Agreement, including such terms as are defined in the Guaranty and Voting Agreement by reference to the Merger Agreement. References to articles and sections in this Amendment refer to such articles and sections of the Guaranty and Voting Agreement.

2. Amendment. The Guaranty and Voting Agreement is hereby amended as follows:

1

3. Binding Effect. Except to the extent expressly provided herein, the Guaranty and Voting Agreement will remain in full force and effect in accordance with its terms. Notwithstanding anything to the contrary, this Amendment does not and will not be construed to modify or amend the limitation of the rights and obligations of the Family Shareholders who are not also Guarantors to Sections 4 through 8 and 10 through 18 of the Guaranty and Voting Agreement, as amended by this Amendment, and the Parties acknowledge that the Family Shareholders who are not also Guarantors are party to this Amendment solely for the purpose of effecting the modification of the Guaranty and Voting Agreement pursuant to this Amendment.

4. Representations and Warranties of the Company. The Company is acting solely at the direction of the Special Committee with respect to the amendment of the Guaranty and Voting Agreement pursuant to this Amendment.

5. Representations and Warranties of The Family Shareholders. Each Family Shareholder has the legal capacity to execute, deliver and perform this Amendment and this Amendment constitutes a legal, valid and binding obligation of such Family Shareholder enforceable against such Family Shareholder in accordance with its terms.

6. Miscellaneous. Sections 8 and 11 through 18 of the Guaranty and Voting Agreement are incorporated herein by reference and, to the extent applicable, will govern the terms of this Amendment.

[Signature Page Follows]

2

IN WITNESS WHEREOF, the Family Shareholders have caused this Amendment to be duly executed and delivered as of the date first written above.

[Signature Page to Amendment No. 1 to Guaranty and Voting Agreement]

[Signature Page to Amendment No. 1 to Guaranty and Voting Agreement]

IN WITNESS WHEREOF, the Guaranteed Party has caused this Amendment to be duly executed and delivered as of the date first written above by its officer thereunto duly authorized.

[Signature Page to Amendment No. 1 to Guaranty and Voting Agreement]