X0306 4 2022-09-01 0 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001655962 York Charles N II 395 OYSTER POINT BLVD, SUITE 217 SOUTH SAN FRANCISCO CA 94080 0 1 0 0 COO, CFO and Secretary Common Stock 2022-09-01 4 S 0 8606 23.1593 D 315795 D Common Stock 2022-09-01 4 S 0 1394 23.7987 D 314401 D The transactions reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan previously adopted by the Reporting Person on September 15, 2021. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $22.7100 to $23.6800 per share, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $23.7100 to $23.8500 per share, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. /s/ Charles N. York II 2022-09-06