The origins of April Fools’ Day (also spelled April Fool’s Day and sometimes called All Fools’ Day) are somewhat obscure. The oldest known prank tradition in the world is Sizdah Bedar, an Iranian holiday that has been celebrated since the sixth century BC on the 13th day of the Persian New Year (Nowruz), which lies on April 1st or 2nd; however, there’s no clear link between this holiday and contemporary April Fools’ Day.
Other precursors or possible influences include the Roman festival of Hilaria, held March 25, and the Medieval Feast of Fools, held December 28 and in turn inspired by the Roman holiday of Saturnalia. There is reference to a French prank holiday being celebrated April 1st in the early 16th century called Poisson d’Avril (literally “April fish”), in which a paper fish is unknowingly attached to the victim’s back (it is still celebrated to some extent in French-speaking countries).
From the Early Middle Ages, up until the late 18th century, many European communities celebrated New Year’s Day on March 25 (Feast of the Annunciation), with some making it a week-long holiday that ended on April 1st. It’s been suggested that April Fools’ Day originated from those who celebrated the new year on January 1st making fun of those who celebrated the alternative festival.