April Fool's Day is celebrated annually on April 1. Here are some facts you probably didn’t know about the unofficial holiday. The exact history of April Fool’s Day, also known as All Fool’s Day, is unknown. Some historians believe that it started in France in the 16th century when the Gregorian calendar was adopted. The Gregorian calendar moved New Year’s Day from April 1 to January 1. Those who continued to celebrate New Year’s on April 1 were known as ‘April Fools’. Others believe that the day originates from the vernal equinox, or the first day of spring in the Northern Hemisphere. They think Mother Nature fools people with changing, unpredictable weather. One April Fool’s Day tradition involved sticking paper fish at the back of gullible people, suggesting that they resemble young fish who get caught or hooked. Another tradition involved sticking signs on people’s backs that say things like “kick me.”