The best “theory” about April Fools’ Day’s origins was itself an April Fools’ Day joke. In 1983, the AP ran a story quoting Joseph Boskin, a history professor at Boston University. He said that April Fools’ Day originated with the emperor Constantine, who allowed a jester to run the empire for one day. While the jester, named Kugel, was emperor, he declared that each year, to commemorate the day he ruled, a day of absurdity should be held. Of course, no such thing ever happened — Boskin was pulling his own April Fools’ Day joke.
There are a number of theories about why we have April Fools’ Day. One of the most common has to do with the switch to the Gregorian Calendar in 1582. Pope Gregory XIII effected this change, moving the start of the New Year to January 1, instead of April 1. During the Middle Ages, the Feast of Annunciation was celebrated on March 25, and festivities often lasted a full week, ending with the beginning of the New Year on April 1. Those who didn’t get the memo (or were staunch traditionalists) were called “April Fools,” and tricks were played on them, with the practice spreading throughout Europe.
As plausible as this explanation sounds, it may not be the origin of April Fools’ Day. References to “April Fools” have been around since before 1582, and the English didn’t even adopt the Gregorian calendar until 1752 — and April 1 tricks were played in that country before the arrival of the change.