The origins of Atheist Day can be traced back to a spoof story that was published on the internet in 2003. Atheist Day was originally about a fictional case of an Atheist who had decided to sue the government.
The reason for the fictional lawsuit was a simple one—unlike all the major religions, there was no day for Atheists, to which the judge said that April 1st (i.e. April Fool’s Day) was their holiday. While this case was just a hoax, the story spread quickly and was actually accepted as fact.
Although the day is best known to be an occasion for pranks, Atheists, in fact, claimed the day for their own as well. And then they did something very their style: they decided to claim it doesn’t actually exist, after all!