An ancient Roman festival and a fish? The theories behind April Fools’ Day are no joke.

April first is pranksters’ paradise: tape over a computer mouse sensor, put food coloring in milk, freeze cereal in the bowl—the ideas for practical jokes are endless. The only thing funnier than April Fools’ Day pranks is perhaps one of the longest running jokes: To this day, no one is sure why we celebrate April Fools’ Day. Here are a few common theories and some background on this mischievous holiday.

When is April Fools’ Day? 

Before getting into the history of April Fools’ Day, it’s important to know when we celebrate it. April Fools’ Day always falls on the first day of April. April Fools’ Day 2024 is on a Monday—so we can add a little extra frivolity to the start of the week. For die-hard pranksters already thinking ahead, it falls on a Tuesday in 2025.

What are the origins of April Fools’ Day? 

Historians have several theories about the origins of April Fools’ Day, but it’s clear that the tradition of prank-pulling is centuries old. 

One possible explanation for the holiday comes from an ancient Roman festival called Hilaria, which was celebrated near the start of April. Hilaria called for all sorts of amusements and games—including masquerades and imitations.

Others believe it can be traced back to 16th-century France, when the switch was made from the Julian calendar—which began the new year in March—to the Gregorian calendar. It is thought that people who were still celebrating the new year in the spring were teased. 

In France, poisson d’avril—or “April fish”—is what you’d call someone who is tricked on April Fools’ Day. The term is first referenced in a poem from 1508. It stems from how fish were more easily caught in the spring, and therefore more “gullible” than any other time of year. 

The origins of April Fools’ Day is so ambiguous that a professor at Boston University, Joseph Boskin, once tricked the Associated Press with his own tale as to how the holiday came about. This hoax dated April Fools’ back to the Roman Empire under Constantine, when a made-up jester named Kugel (inspired by the Jewish noodle dish) was made king for a day.

The first definitive reference to April Fools’ Day came in 1561, in a Flemish poem by Eduard de Dene. The poem tells of a nobleman sending his servant on fruitless errands. The servant recognized that, because it was April 1st, he was being sent on fool’s errands. (Sending someone to find a non-existent item or person is still a popular prank, so if you’re brainstorming how to fool friends and family this year, look no further.)