Every April 1st, the world is divided into two camps— the pranksters and their hapless victims. April Fools' Day, believe it or not, began with 16th century Europe, which celebrated the New Year on the Feast of the Annunciation. Starting on March 25th, a week's worth of celebration would climax on April 1st and the New Year would then begin in earnest.
But in 1582, Pope Gregory XIII ordered a new calendar to be constructed, and shifted New Year's Day to January 1st. Those who continued to celebrate the New Year on April 1st were considered to be unsophisticated. A tradition rose around making these "April fools" seem even more foolish by subjecting them to tricks and pranks. And thus, April Fools' Day was born.