April Fool's Day origins are uncertain but possibly stem from a change in calendars or the arrival of spring. In France, April 1st is called Poisson d'Avril where kids tape a paper fish to each other's backs and shout "Poisson d'Avril" when discovered. While celebrated in some countries like the US and France, April Fool's tricks should be harmless and make the victim at least smile when the prankster reveals the joke.