French Calendar Reform
A popular legend holds that April Fool's Day began in 1564 when the 14-year-old King Charles IX of France passed the Edict of Roussillon
, decreeing that henceforth the year would begin on January 1, instead of April 1. Supposedly some people failed to hear about the change and continued to celebrate the new year on April 1, causing people to make fun of them and play pranks on them. In this way, they became the first April fools.
The existence of references to April Fool's Day before 1564 suggests that this cannot have been the origin of the celebration. But also, the story is incorrect in that April 1 was not considered the start of the year in any part of France before 1564. The year began on different days in different parts of the country (March 25, March 1, Easter, or January 1). One of the reasons for the reform was to impose a single calendrical system throughout the entire country.