Why do French people stick a fish on people's back for April Fools' Day?

The most common belief is that the observance of April Fool's Day began in France after the adoption of the reform calendar by Charles IX in 1564.

When New Year's Day was changed to January 1st people who still celebrated New Year's Day on April 1st were the 'butt' of many jokes. They were called "Poisson d'Avril" (April Fish) because at that time of year the sun was in the zodiac sign of Pisces, the fish.

In France it became customary to play tricks on family and friends. French children fool their friends by taping a paper fish to their friends' backs and when the "young fool" discovers this trick, the prankster yells "Poisson d'Avril!"