For pranksters, no holiday tops April Fools' Day. Celebrated annually on April 1, it is the one day of the year when lighthearted tricks are not just accepted — they are welcomed. The origin of this fun tradition is unclear. Some believe it began in 1582 when the world moved from the Julian calendar — which started the new year around April 1 — to the Gregorian calendar. Others think it began as a way to celebrate the arrival of spring.