France
The French origin theory (the calendar-change hypothesis) was discussed above. It alleges that the custom originated when King Charles IX reformed the calendar, moving the start of the year from April 1 to January 1. People who continued to celebrate New Years on April 1 were mocked and had pranks played on them, thus initiating the custom of April 1st foolery. This has become, worldwide, the most popular theory of the origin of April Fool’s Day, despite its flaws.

The French also have a theory that traces the origin of the custom back to the abundance of fish to be found in French streams and rivers during early April when the young fish had just hatched. These young fish were easy to fool with a hook and lure. Therefore, the French called them ‘Poisson d’Avril’ or ‘April Fish.’ Soon it became customary (according to this theory) to fool people on April 1, as a way of celebrating the abundance of foolish fish. The French still use the term ‘Poisson d’Avril’ to describe April Fool’s Day pranks. They also observe the custom of giving each other chocolate fish on April 1.

http://www.museumofhoaxes.com/hoax/Hoaxipedia/April_Fools_Day_-_Origin/