The most popular theory attributes the day to 16th-century France. When King Charles IX moved New Year's from the end of March to Jan. 1, those who kept celebrating in spring were mocked and called fools. Another theory ties the tradition to the ease with which newly hatched fish could be caught in early April. Fooling people on April 1 became a way of celebrating the abundance of "foolish" fish. The French still call April Fool's pranks Poisson d'Avril, or April Fish.
An important meeting of German lawmakers in Augsburg was scheduled for April 1, 1530, but was canceled. Some citizens had wagered that the meeting would occur; they lost their money and were mocked for their foolishness, setting off the tradition of joking on the first of April.
British legend traces the celebration to a town called Gotham. Residents prevented the king from passing through town because any road he traveled became public property. The king sent his Army to punish them, but the soldiers found the citizens behaving like lunatics. The clever townspeople were only putting on an act, but the king decided they were too foolish to be punished.
The Dutch gained their independence from Spain after nearly a century of war. In one critical battle on April 1, 1572, Dutch rebels captured the town of Den Briel. They began commemorating the humiliation of the Spanish forces with a tradition of pulling pranks on the anniversary of the battle.
When Constantine ruled the Roman Empire, a group of jesters persuaded him to make one of their own king for a day. Constantine agreed and made a fool, called Kugel, king on April 1. During his brief reign, the jester proclaimed that the day would be one of absurdities and hoaxes, hence the origins of the celebration. note: this theory was itself an April Fool's day hoax by Boston university professor Joseph Boskin in 1983. He fooled an associated press reporter into believing it, and the story was published widely before the news organization realized its mistake.