I know #ActivityPub does not exactly have an official holiday, but on January 23rd (2018) the WC3 gave their official recommendation of ActivityPub, which I believe helped it gain popularity amongst numerous social platforms.

👉🏾 Happy #Fediverse Day! darnell.day/happy-fediverse-da

So if you can, raise a glass of gratitude to the five founders of the Fediverse (listed in the link), as without them the masses would lack credible alternatives to the major social networks that exist today.