The origins of April Fools' Day are murky, but the likeliest explanation is that it began as a way to mock French people who were slow to switch to the Gregorian Calendar which changed New Year's from April 1 to January 1. These folks were labeled "fools" and some were sent on "fools' errands."
Of course there are alternate theories, specifically ones that ascribe the informal holiday to the cultural impact of the Hilaria Festival of ancient Rome, held on March 25, and the Holi celebration in India, which ends on March 31.
The Museum of Hoaxes has a complete collection of all the theories, but the real problem with explaining April Fools' Day is that you never quite know when someone is trying to fool you with their explanation. The classic example of this comes from Joseph Boskin: Constantine and Kugel. This Boston University professor explained that the holiday stemmed from a moment of political unrest under Constantine, when a group of court jesters said they could run the empire better than he could. He claimed that Constantine was amused so he let a jester named Kugel be king for a day...April 1. The "AP" ran this theory in 1983, only to find out it was Boskin's prank on the American public.
The real fun, as we all know, is executing these pranks ourselves or, at the very least, reveling pranks past. Here is our collection of the best pranks of all time...