For pranksters, there is no better holiday than April Fools' Day. Celebrated annually on April 1, it is the only day of the year when fun, harmless hoaxes go unpunished. The mischievous holiday has murky origins. Some believe it started in 1582 when France transitioned from the Julian calendar — which began the year around April 1 — to the currently used Gregorian calendar. Those unaware, or unwilling, to accept the date change were pranked. Others think the holiday started as a cheerful way to mark the start of spring.