April Fools' Day or All Fools' Day, though not a holiday in its own right, is a notable day celebrated in many countries on April 1. The day is marked by the commission of hoaxes and other practical jokes of varying sophistication on friends, enemies and neighbors, or sending them on fools' errands, the aim of which is to embarrass the gullible. Traditionally, the jokes only last until noon. If you play a trick on someone after this time you are the April Fool.
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http://en.wikipedia.org/wiki/April_Fools'_Day