In France, the tradition on what we would call April Fool's Day is called Poisson d'Avril. The aim of the game is to pin or stick an 'April fish' onto as many unsuspecting colleagues, friends and family as possible.
Last year our laitière's small daughter, who often accompanies her mother on the milk round, entertained herself by drawing fish on postit notes and sticking them to the plastic bags of milk. That allowed her to shout 'Poisson d'avril!' as I took delivery.