April Fool's Day is celebrated on April 1. There are numerous theories about the origin of this day, but the most commonly accepted is that the first April Fools' Day was in 1582. That was the year that France adopted the Gregorian Calendar, which moved the observance of New Year's from the end of March (vernal equinox), to January 1. Some people continued to celebrate the New Year on April 1, and their perceived foolishness spawned pranks.