April first: the special day when some pull out all the stops to pull the biggest pranks, and others fall victim to their schemes. But, why do we celebrate this holiday of chaos, and where did its wacky traditions come from? That remains largely a mystery, but historians have a few ideas from different cultures.

The most popular theory centers around the Council of Trent dictating a switch from the Julian calendar system to the Gregorian calendar system. This change shifted the start of the year from the spring equinox on April first to January first. France adopted this measure in 1582, but not everyone got the message, with some continuing to celebrate the new year during the last week of March. These people were dubbed “April fools” and became the butt of jokes where paper fish referred to as “poisson d’avril” (April fish), said to symbolize a young, easily caught fish and a gullible person, were placed on their backs.

Some other fun theories about the start of this humorous holiday include that it was tied to the first day of Spring in North America, where Mother Nature played tricks on people with the weather, and the ancient Roman festival of Hilaria, where followers of the cult of Cybele dressed up and mocked other citizens.