The obvious choice for ActivityPub’s birthday would be the 23rd of January 2018 - the day it was annointed as a W3C recommendation. That doesn’t seem quite right though - its not as if the spec came into existence in any sense upon that date. In fact, Mastodon implemented it before thne.
There are several possible dates you might pick, but for me it will always be September 5th 2014 - when I committed the first sketch of a specification I called ActivityPump [github.com] and pushed it to Github
It wouldn’t be until November that I actually submitted (a revised and enhanced version of) that draft to the working group, but even then I had the very nucleus of the specification written down.
Happy 10th birthday, ActivityPub. 🍰