In all seriousness, April Fool's day actually IS National Atheist's Day, according to this urban tale (which may or may not be true):
In Florida, an atheist created a case against the Easter & Passover holy days. He hired an attorney to bring a discrimination case against Christians, Jews & observances of their holy days. The argument was, it was unfair that atheists had no such recognized day(s). The case was brought before a judge. After listening to the passionate presentation by the lawyer, the judge banged his gavel declaring, 'Case dismissed.'
The lawyer immediately stood objecting to the ruling saying, 'Your honor, how can you possibly dismiss this case? The Christians have Christmas, Easter & others. The Jews have Passover, Yom Kippur & Hanukkah. Yet my client & all other atheists have no such holidays.'
The judge leaned forward in his chair saying, 'But you do. Your client, counsel, is woefully ignorant.'
The lawyer said, 'Your Honor, we are unaware of any special observance or holiday for atheists.'
The judge said, 'The calendar says April 1st is 'April Fools Day.' Psalm 14:1 states, 'The fool says in his heart, there is no God.' Thus, it is the opinion of this court, that if your client says there is no God, then he is a fool. Therefore, April 1st is his day. Court is adjourned.