In France, April Fools' Day is known as “Poisson d'Avril,” meaning “April fish.”
The French custom combines practical jokes with the attempt to attach a paper fish to someone’s back without being noticed. Once discovered, the victim may be told “Poisson d’Avril!” The phrase can refer both to the day’s prank tradition and to the person caught by it.
France is the clearest canonical answer for a quiz asking where the expression is used, although the same French term is also familiar in French-speaking parts of Belgium, Switzerland, and Canada. Italy has a related expression, pesce d'aprile, while Dutch uses aprilvis, so the tradition is broader than France alone.
The phrase has older literary roots than the modern calendar-change explanation often given for April Fools’ Day. References to poisson d’avril appear in French sources from the 15th and 16th centuries. The precise origin remains debated, with theories involving spring customs, fishing seasons, Lent, and older festivals of role reversal.