From France Travel Guide:  Although the origins of April Fools is obscure and debated, the most widely accepted explanation actually credits the “holiday” as starting in France. The most popular theory about the origin of April Fool’s Day involves the French calendar reform of the sixteenth century.

The theory goes like this: In 1564 King Charles XIV of France reformed the calendar, moving the start of the year from the end of March to January 1. However, in a time without trains, a reliable post system or the internet, news often traveled slow and the uneducated, lower class people in rural France were the last to hear of and accept the new calendar. Those who failed to keep up with the change or who stubbornly clung to the old calendar system and continued to celebrate the New Year during the week that fell between March 25th and April 1st, had jokes played on them. Pranksters would surreptitiously stick paper fish to their backs. The victims of this prank were thus called Poisson d’Avril, or April Fish—which, to this day, remains the French term for April Fools—and so the tradition was born.

Today in France, those who are fooled on April 1 are called the “Poisson d’Avril” (the April Fish). A common prank (especially among school-aged children) is to place a paper fish on the back of an unsuspecting person. When the paper fish is discovered, the victim is declared a “Poisson d’Avril.”

While it is not clear of the origins of fish being associated with April 1, many think the correlation is related to zodiac sign of Pisces (a fish), which falls near April.  Continuez ici.