April Fools’ Day is an all-day celebration of laughs and practical jokes. Throughout modern history, gags have been played out on April 1, both little and large scale. But the greatest prank of all might very well be the day itself: no one is quite sure where and when April Fools’ Day began!

Below are a few theories on where the hilarious holiday might have originated.

Theory #1: A Shift in the Calendar.

The Council of Trent was held in Northern Italy from 1545 to 1563. Pope Gregory XII decreed that everyone was to follow the Gregorian calendar from now on – instead of Julius Caesar’s Julian calendar. The new Gregorian calendar began on January 1. But those still using the Julian calendar celebrated New Years’ the last week of March all the way up to April 1. It is said that the news spread so slowly of the different new year that those still celebrating in March were made fun of for not being “in the know”. Soon the pranks followed. They were marked as “April fools.”

Theory #2: The Hilaria Festival.

In Ancient Rome, there was a festival known as Hilaria. This day was full of masks, games, and jokes, and was typically celebrated on March 25, all to honor Cybele, the mother of the gods. There were days before and after the 25th that each held significance, but could all this partying be the reason for our April Fools’ Day?

Theory #3. The Feast of Fools.

Celebrated in medieval France and England, the Feast of Fools was a carnival party. Townspeople were encouraged to dress up like those opposite of their social class. Workers dressed like royalty, nobles like peasants, etc. Soon the celebration got out of hand and those in charge banned celebrating the Feast of Fools any longer, which proved to not be an easy task. It was hundreds of years before people stopped commemorating the holiday, even after the official ban.

Theory #4. Scotland’s “Hunting the Gowk”.

In 18th century Scotland it is recorded that a two-day tradition began in which people picked those they wanted to trick, sending them out on phony errands as a joke. “Gowk” was a word they used to describe a cuckoo bird, or a symbol for a foolish person.

Theory #5. Mother Nature Herself.

The Vernal Equinox, or the first day of the meteorological season of spring, might very well be the reason for April Fools’ Day. Throughout history in the Northern Hemisphere, Mother Nature has always seemed to “fool” everyone with her wild, unpredictable weather.

How April Fools’ Day got its start can be a fun historical debate. But one thing is for sure: be prepared for major silliness when you see April 1 on the calendar!

References: time.com, history.com, washingtonpost.com