The article enstablishing when a new edition of the code becomes effective is ASME Sec. I, Mandatory Appendix VI "Enstablishing governing code editions, addenda, and cases for boilers and replacement parts", paragraph VI-1 "General":
After Code revisions are approved by ASME, they may be used beginning with the date of issuance shown on the code. Except as noted below,revisions become mandatory 6 months after the date of issuance.
Code therefore can be used starting its publishing date, for example July 1st, 2021 for the 2021 edition.
It becomes mandatory six months after said date, for example January 1st, 2022 for the 2021 edition.