For people who enjoy pranks, there is no better holiday than April Fools' Day. Celebrated annually on April 1, it is the only day of the year when light-hearted tricks are not frowned upon. The origin of this fun tradition is hazy. Some maintain that it started in 1582 when the world moved from the Julian calendar — which began the new year in April — to the current Gregorian calendar. Others think it was to celebrate the onset of spring.