Happy Poisson d'Avril

I found a fairly complicated explanation:

The origins of “April Fish Day” in France date back to medieval times. One theory revolves around the fact that until 1564, New Year’s Day was celebrated on March 25 in Europe. (Which makes sense–spring is a time of new beginnings, so why wouldn’t it mark the start of the new year?) By some accounts, these celebrations would last an entire week, culminating on April 1st. But then came the Edict of Rousillon, in which King Charles IX announced a new calendar, what we now know as the Gregorian calendar, which would have the new year start on January 1st. The story goes that some people refused to celebrate the new year in January, and continued to celebrate at the start of spring. Those who clung to tradition instead of adapting to the times became the subjects of ridicule, and ended up on the receiving end of many a joke. People would invite them to parties that didn’t exist, or send them gag gifts.

But references to Poisson d’Avril and April Fools predate this event by quite some time. A 1508 poem by French poet Eloy D’Amerval called Le Livre de la Deablerie (The Book of Deviltry) contains what is thought to be the first reference to a “Poisson d’Avril.” The poem includes a line that reads: “Maquereau infâme de maint homme et de mainte femme, poisson d’avril” (“infamous mackerel of many a man and woman, the April fish&rdquo. It’s not specified in the poem if the “Poisson d’Avril” refers to April 1st in particular, as it could just refer to a foolish person.