Hriston
Dungeon Master of Middle-earth (He/him)
According to Wikipedia, the 5th Ed. SRD was released in January 2016 and updated to SRD 5.1 in May 2016. There don't seem to be any rules changes between the two versions (the update had to do with the inclusion of certain IP and the term DM, if I recall correctly), so I think the publication of Curse of Strahd (March 15, 2016) is actually a really good event to mark the beginning of 5.1. CoS was the first adventure for the edition that was produced entirely in-house, whereas earlier adventure books were the result of collaborations with other studios or produced under commission. So I'd categorize 5E's publication history (so far) into the following two "eras":5.2 is this SRD revision. SRD v5.0 was 2015. SRD v5.1 was actually 2016's revisions around the release of Curse of Strahd.
5.0 (July 15, 2014 - January, 2016): This period commences with the publication of the Starter Set, followed by the core rulebooks and "Tyranny of Dragons" pair of adventures in 2014, continues with more adventures and Sword Coast Adventurer's Guide produced with assistance from other game studios in 2015, and culminates with the publication of the SRD (5.0) in January 2016.
5.1 (March 15, 2016 - present): This period is characterized by mostly in-house production of adventures and setting materials, a reliance on anthology books to round out the adventure publishing schedule, and a proliferation of supplemental rules expansions.
5.2 is set to begin on September 17, 2024 with the publication of the first of the revised core rulebooks.