While EA doesn't have any widely-celebrated celebrated holidays, there are some days centred around EA causes, influential people, or humanity's achievements that may be worth calling attention to in your group. These days can serve as good educational opportunities, and contribute to the sense of connection with the broader EA and social impact community.
Here are some ideas of how you might celebrate some of these dates:
Smallpox eradication day: read Will MacAskill article
Petrov Day: Screen The Man Who Saved the World
Peter Singer: watch a Peter Singer talk?
Malaria Day: (videos)
You can celebrate many of these events (World Malaria Day, World Forest Day, World Vegetarian Day, etc.) by holding a fundraiser
Have a trivia night with questions around that topic
Watch a documentary on that topic
These are also a good opportunity to advertise on social media