This answer can possibly help you: reference
As per its naming convention Summer '20 (latest), Spring '20 and so on, Salesforce seems to begin its primary release when Summer begins, and then adds features/fixes throughout the Summer.
If you open the above answer link, and open the PDFs. Open the Release Notes section and scroll all the way down to last release note. That will be the beginning of the initial release of Summer '20.
Whether you want final stable release date or primary release date, you can extract as per your use case.
[EDIT] For eg: If you want the release date for Summer '20 release, final stable release date is July 8, 2020 (and it may continue as until Summer ends) and primary release date is May 4, 2020.