April Fools' Day or All Fools' Day is a day celebrated in various countries on April 1. The day is marked by the commission of hoaxes and other practical jokes of varying sophistication on friends, family members, enemies, and neighbors, or sending them on a fool's errand, the aim of which is to embarrass the gullible. Traditionally, in some countries, such as the UK, Australia, and South Africa the jokes only last until noon, and someone who plays a trick after noon is called an "April Fool". Elsewhere, such as in France, Ireland, Italy, South Korea, Japan, Russia, The Netherlands, Brazil, Canada, and the U.S., the jokes last all day.
Iranians play jokes on each other on the 13th day of the Persian new year (Norouz), which falls on April 1 or April 2. This day, celebrated as far back as 536 BC, is called Sizdah Bedar and is the oldest prank-tradition in the world still alive today; this fact has led many to believe that April Fools' Day has its origins in this tradition.
The April 1 tradition in France and French-speaking Canada includes poisson d'avril (literally "April's fish"), attempting to attach a paper fish to the victim's back without being noticed. This is also widespread in other nations, such as Italy (where the term Pesce d'aprile (literally "April's fish") is also used to refer to any jokes done during the day). In Spanish-speaking countries, similar pranks are practiced on December 28, día de los Santos Inocentes, the "Day of the Holy Innocents". This custom also exists in certain areas of Belgium, including the province of Antwerp. The Flemish tradition is for children to lock out their parents or teachers, only letting them in if they promise to bring treats the same evening or the next day.
Taken from: wikipedia