Back in 1582, France switched from the Julian calendar to the Gregorian calendar causing New Year’s Day to move from April 1st to January 1.
This was decreed by Pope Gregory XIII fopr all European countries.
Some believe the tradition of April Fools Day began when people continued to ring in the New Year on April 1st because they hadn’t heard about the calendar change.
These fools were laughed at by others and had tricks playing on them including paper ‘Poisson d’Avril’ (April Fish) fish stuck to their backs, symbolising being gullible like young, easily caught fish. So… now we celebrate april fool on april 1