April Fools’ Day, which falls on April 1st each year, has been celebrated for centuries by various cultures, though its exact origins are unknown. Traditions associated with April Fools’ Day include playing hoaxes or practical jokes on others, often followed by shouting “April Fools!” to reveal the prank. Despite its mysterious history, the media and major brands embracing April Fools’ Day jokes have ensured the unofficial holiday’s longevity.

Some historians speculate that April Fools’ Day dates back to 1582 when France switched from the Julian calendar to the Gregorian calendar as called for by the Council of Trent in 1563. In the Julian Calendar, as in the Hindu calendar, the new year began with the spring equinox around April 1.

People who were slow to get the news or failed to recognize that the start of the new year had moved to January 1 and continued to celebrate it during the last week of March through April 1 became the butt of jokes and hoaxes and were called “April fools.” These pranks included having paper fish placed on their backs and being referred to as “poisson d’avril” (April fish), which is said to symbolize a young, easily caught fish and an unsuspecting person.

Historians have linked April Fools’ Day to ancient festivals such as Hilaria (Latin for joyful), which was celebrated in ancient Rome at the end of March by followers of the cult of Cybele. During this festival, people would disguise themselves and mock fellow citizens and even magistrates. This celebration was said to be inspired by the Egyptian legends of Isis, Osiris, and Seth.

There’s also speculation that April Fools’ Day was tied to the vernal equinox or the first spring day in the Northern Hemisphere when Mother Nature fooled people with changing, unpredictable weather.