April Fool's SEEMS perfect, as atheists probably shouldn't hold anything as HOLY.
(I'm such a bitch)
April Fool's day does have an origin in religion. And it's Catholic. (Groan).
It has something to do with the silly peasants not adopting our new shiny Catholic "right" calendar, and still celebrating New Year's on April First. That silly silly secular peasant (known as...ha ha, a "pagan," was given a fish, because he is a Fish Fool. "Poisson d'Avril!! Ho Ho Ho," says the Frenchie.
Now that I think about it, it makes PERFECT SENSE! Atheists CAN celebrate New Year's Day, or Atheist's Day on April Fools'! It's a rejection of the Gregorian calendar, AND christians can point and say "THE FOOL IN HIS HEART SAYS THERE IS NO GOD BLAH BLAH BLAH!" See? It pleases everyone.
I'm not a big fan of having it be April Fool's Day for a few reasons:
1. It's still pretty cold around here on April 1 and if I'm going to support a new holiday, it's going to be one where I can go swimming and pig out on brats fresh off the grill and drink lots of beer to keep cool.
2. The best holidays, and we want ours to be the best, are always on Fridays or Mondays, because they expand the weekend. April 1 is going to fall on a different day from year to year.
3. We atheists are usually an independent-minded bunch, so getting a consensus on when to place a holiday would probably be impossible. I think smaller local atheist groups would do a better job of finding agreement on when and where to schedule Atheist celebrations.
IMHO, everyday is a good day to celebrate atheism, because everyday we survive is a small victory against those who are still waiting for the world to end.