A NoGodBlog post from a couple of days ago (4/13) mentioned that it was Atheist Day! I had no idea there was one (that wasn't a joke). I can't find much information about it, but the post indicates that it is due to it being Madelyn Murray O'Hair's and Thomas Jefferson's birthday. Ok, so that's a neat concept -- Thomas Jefferson is one of my favorite atheists people that don't believe in a personal God. I'll try to remember this one next year.
Some theists call April 1st National Atheist's Day. Get it? It's a pun on April Fool's day and the biblical version "The fool hath said in his heart, There is no God." (Psalm 14:1)
There is also the National Day of Reason, the 1st Thurs. in May every year. That is one I will definitely be celebrating this year! It's meant to coincide with the National Day of Prayer. And if any theists get the idea that it's not respectful to land our day right on top of their, just remind them that they landed Easter right on top of Eostre, and Christmas almost right on top of Winter Solstice.
I think having a National Day of Reason is good, but I would like to see a little more connection being made between the day and the atheists behind it. If nobody understands that reason=atheism, then the publicity of having a holiday for us isn't really going to do any good.