There are quite a few "theories", but not a lot of actual facts surrounding the history of April Fools' Day (also known as All Fools' Day). What is factual? Well, the first day of April, although not a national or worldwide "recognized" holiday, is still a time of year were many countries share light-hearted jokes or pranks amongst the population.

One possible account of April Fools' lore, is during the late 1500s, Pope Gregory XIII ordered a new calendar (the Gregorian Calendar, which is the one we use today) to replace the old Julian Calendar. The new calendar called for New Year's Day to be celebrated January 1st. That year, France adopted the reformed calendar. The popular explanation is that many people refused to accept the new date, or more likely, were not privy to the change, and thus continued to celebrate New Year's Day on April 1st. Other people began to make fun of these traditionalists, sending them on "fool's errands" or trying to trick them into believing something false. Eventually, the practice spread throughout Europe.

Although not a fact, a decent explanation. Of course, troll the internet for a few minutes and you're likely to come across several more. One thing is for sure... April Fools' is here to stay.