UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 3)*

Day One Biopharmaceuticals, Inc.

(Name of Issuer)

Common Stock, par value $0.0001 per share

(Title of Class of Securities)

23954D109

(CUSIP Number)

Alejandro Moreno

Langhorne S. Perrow

c/o Access Industries, Inc.

40 West 57^th Street, 28th Floor

New York, New York 10019

(212) 247-6400

with copies to:

Nicholas P. Pellicani

Debevoise & Plimpton LLP

65 Gresham Street

EC2V 7NQ

London

+44 20 7786 9000

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

October 18, 2023

(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 Rules 13d-1(e), 13d-1(f) or 13d-1(g), check the following box. ☐

CUSIP No. 23954D109

CUSIP No. 23954D109

CUSIP No. 23954D109

CUSIP No. 23954D109

CONTINUATION PAGES TO AMENDMENT NO. 3 TO SCHEDULE 13D

This Amendment No. 3 to Schedule 13D is being filed by AI Day1 LLC (“AI Day1”), Access Industries Holdings LLC (“AIH”), Access Industries Management, LLC (“AIM”) and Len Blavatnik (collectively, the “Reporting Persons”, and each, a “Reporting Person”), in respect of the common stock, par value $0.0001 per share (the “Common Stock”), of Day One Biopharmaceuticals, Inc. (the “Issuer”).

The Schedule 13D filed with the Securities and Exchange Commission (“SEC”) on June 9, 2021, as amended by supplemented by Amendment No. 1 to the Schedule 13D filed by the Reporting Persons with the SEC on June 21, 2022 and Amendment No. 2 to the Schedule 13D filed by the Reporting Persons with the SEC on June 12, 2023 (together, the “Schedule”) is hereby amended and supplemented by the Reporting Persons as set forth below in this Amendment No. 3. This amendment is filed by the Reporting Persons in accordance with Rule 13d-2 of the Securities Exchange Act of 1934, as amended, and refers only to information that has materially changed since the filing of the Schedule. The items identified below, or the particular paragraphs of such items which are identified below, are amended as set forth below. Unless otherwise indicated, all capitalized terms used and not defined herein have the respective meanings assigned to them in the Schedule.

Item 3 Source and Amount of Funds or Other Considerations

The disclosure in Item 3 is hereby supplemented by adding the following at the end thereof:

On October 18, 2023, AI Day 1 purchased 979,667 shares of Common Stock at a weighted average price of $10.2076 per share in open market transactions.¹ AI Day 1 funded these purchases using capital contributed from affiliated entities, which funded that capital using cash on hand.

On October 19, 2023, AI Day 1 purchased 384,400 shares of Common Stock at a weighted average price of $10.0608 per share in open market transactions.² AI Day 1 funded these purchases using capital contributed from affiliated entities, which funded that capital using cash on hand.

On October 20, 2023, AI Day 1 purchased 111,387 shares of Common Stock at a weighted average price of $9.9734 per share in open market transactions.³ AI Day 1 funded these purchases using capital contributed from affiliated entities, which funded that capital using cash on hand.

Item 5 Interest in Securities of the Issuer

The disclosure in Item 5(a), (b) (and (c) is hereby amended and restated as follows:

(a) and (b) The responses of each of the Reporting Persons with respect to Rows 11, 12, and 13 of the cover pages of this Schedule 13D that relate to the aggregate number and percentage of common stock (including but not limited to footnotes to such information) are incorporated herein by reference.

The responses of each of the Reporting Persons with respect to Rows 7, 8, 9, and 10 of the cover pages of this Schedule 13D that relate to the number of common stock as to which each of the persons or entities referenced in Item 2 above has sole or shared power to vote or to direct the vote of and sole or shared power to dispose of or to direct the disposition of (including but not limited to footnotes to such information) are incorporated herein by reference.

12,929,322 shares of Common Stock are owned directly by AI Day1 and may be deemed to be beneficially owned by AIH, AIM and Mr. Blavatnik because (i) AIH indirectly controls all of the outstanding voting interests in AI Day1, (ii) AIM controls AIH and (iii) Mr. Blavatnik controls AIM and controls a majority of the outstanding voting interests in AIH. Each of the Reporting Persons (other than AI Day1), and each of their affiliated entities and the officers, partners, members and managers thereof, disclaims beneficial ownership of these securities.

(c) The following transactions in the Issuer’s securities have been effected by Reporting Persons within the 60 days prior to this filing:

The information set forth in Item 3 of this Schedule 13D is incorporated by reference herein.

Item 7 Materials to Be Filed as Exhibits

The disclosure in Item 7 is hereby supplemented by adding the following at the end thereof:

SIGNATURE

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

Date: October 20, 2023

Exhibit 99.9

Joint Filing Agreement

The undersigned hereby agree that they are filing this statement jointly pursuant to Rule 13d-1(k)(1) promulgated under the Securities Exchange Act of 1934, as amended. Each of them is responsible for the timely filing of such Schedule 13D and any amendments thereto, and for the completeness and accuracy of the information concerning such person contained therein; but none of them is responsible for the completeness or accuracy of the information concerning the other persons making the filing, unless such person knows or has reason to believe that such information is inaccurate.

In accordance with Rule 13d-1(k)(1) promulgated under the Securities Exchange Act of 1934, as amended, the undersigned hereby agree to the joint filing with each other on behalf of each of them of such a statement on Schedule 13D (and any amendments thereto) with respect to the Common Stock owned by each of them, of Day One Biopharmaceuticals, Inc., a corporation incorporated under the laws of the State of Delaware. This Joint Filing Agreement shall be included as an exhibit to such Schedule 13D.

[Signature Page Follows]

IN WITNESS WHEREOF, the undersigned hereby execute this Joint Filing Agreement as of the 20^th day of October, 2023.