Although April Fools’ Day, also called All Fools’ Day, has been celebrated for several centuries by different cultures, its exact origins remain a mystery.

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.

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.

These pranks included having paper fish placed on their backs and being
referred to as “poisson d’avril” (April fish), said to symbolize a young, easily
caught fish and a gullible person.