Poisson d'avril
Poisson d'avril is held on April 1. France, Switzerland, French Community of Belgium. This event in the first decade of the month April is annual. 
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An April Fool's joke is a joke or prank that you play on April 1st on your acquaintances, friends and family. It is also customary to make hoaxes in the media, both written press, radio, television and on the Internet (computer hoax).
Another April 1st joke is to hang a fish (often made of paper) on the back of a person you want to make fun of without them realizing it. "April Fool!" is an exclamation that is said once one of the jokes is discovered.
According to folklorist Alan Dundes, several usages seem to have actually blended with that of carnival on this day, which is a likely outgrowth of ancient fertility and reversal rites, celebrations in which victims of pranks are confronted with a calendar rite of passage: influence of ancient festivals, exit of the zodiacal sign of Pisces (last sign of winter and last zodiacal sign), April considered in the Middle Ages as the first month of spring (April 1 was set as a spring rite), extension the period of Lent when it is allowed to eat only fish.
Similar holidays and events, festivals and interesting facts
Tree Planting Day in Belgium, Italy, Lesotho, Portugal on March 21
World Office Storage Day on March 21
World Party Day on April 3
Näfelser Fahrt in Glarus, Switzerland on April 3 (The Battle of Näfels was fought on 9 April 1388 between Glarus with its allies the Old Swiss Confederation, and the Habsburgs. It was a decisive Glarner victory);
AXA Day on April 8 (AXA Group);
Malbec World Day on April 17 (Malbec is a purple grape variety used in making red wine. MWD is celebrated to commemorate the day when president Domingo Faustino Sarmiento of Argentina officially made it his mission to transform Argentina's wine industry. On that day, back in 1853, he tasked Michel Aimé Pouget, a French soil expert, to bring over new vines);
Easter in Western Christianity on April 20 (Første påskedag in Norway)