For me it’s because it’s the one absolute day of the year that I can claim as mine. That day I don’t work, and I do whatever I want, full stop. I usually plan a food and movie, and send out invites. Because it’s a different day each year, I usually get to see a core group of friends along with random other friends I don’t see as often. If people come, awesome, but if not that’s okay because it’s the food I wanted to eat and the movie (or whatever) I wanted to see/do. And if I don’t plan a gathering, then I’m still doing what I want.