For pranksters, there is no better holiday than April Fools' Day. Celebrated every April 1, it is the one day of the year when harmless tricks are not just allowed — they are expected. Some historians believe the fun tradition began in 1582, when Europe switched from the Julian calendar to the Gregorian calendar. Today, April Fools' Day is celebrated all over the world. Even companies join in the fun with clever pranks that often trick people for hours — and sometimes even days.