April fool

[WillSmith]

Where does 'April Fool's Day' come from?

[Tophat]

April Fools Day, also called All Fools Day, first day of April, named from the custom of playing practical jokes or sending friends on fools errands on that date. Although it has been observed for centuries in several countries, the origin of the custom is unknown.(Britannica)

[rekstout]

The change to the Gregorian calendar in 1582 in France moved new years day from 25th March to 1st January. Prior to the change there was a week long celebration of the new year ending on April 1st. Some (mostly rural) regions did not take up this change immediately and became the butt of jokes and pranks as a result. The term April Fool could be a corruption of the French term used at the time of Poisson d'Avril (April Fish) (Source - Focus Magazine May 2001)