So far my favorite explanation of April Fools Day is from the following:
Another explanation of the origins of April Fools' Day was provided by Joseph Boskin, a professor of history at Boston University. He explained that the practice began during the reign of Constantine, when a group of court jesters and fools told the Roman emperor that they could do a better job of running the empire. Constantine, amused, allowed a jester named Kugel to be king for one day. Kugel passed an edict calling for absurdity on that day, and the custom became an annual event.
"In a way," explained Prof. Boskin, "it was a very serious day. In those times fools were really wise men. It was the role of jesters to put things in perspective with humor."
This explanation was brought to the public's attention in an Associated Press article printed by many newspapers in 1983. There was only one catch: Boskin made the whole thing up. It took a couple of weeks for the AP to realize that they'd been victims of an April Fools' joke themselves.
April Fools' Day - Origin and History