April Fools' Day is celebrated in different countries around the world on April 1 every year. Sometimes referred to as All Fools' Day, April 1 is not a national holiday, but is widely recognized and celebrated as a day when many people play all kinds of jokes and foolishness. The day is marked by the commission of good-humoured or otherwise funny jokes, hoaxes, and other practical jokes of varying sophistication on friends, family members, teachers, neighbors, work associates, etc.

Traditionally, in some countries such as Canada, New Zealand, the UK, Australia, Cyprus, and South Africa, the jokes only last until noon, and someone who plays a trick after noon is called an "April Fool" and taunted "April Fool's Day's past and gone, You're the fool for making one." Elsewhere, such as in France, Italy, South Korea, Japan, Russia, The Netherlands, Germany, Brazil, Ireland, and the U.S., the jokes last all day. In France and Italy children (and adults, when appropriate) traditionally tack paper fish on each other's back as a trick and shout "april fish!" in their local language ("poisson d'avril!" and "pesce d'aprile!" in French and Italian respectively).

The earliest recorded association between April 1 and foolishness can be found in Chaucer's Canterbury Tales (1392). Many writers suggest that the restoration of January 1 as New Year's Day in the 16th century was responsible for the creation of the holiday, but this theory does not explain earlier references.