The origins of April Fools' Day are obscure. The most commonly cited theory holds that it dates from 1582, the year France adopted the Gregorian Calendar, which shifted the observance of New Year's Day from the end of March (around the time of the vernal equinox) to the first of January.
According to popular lore some folks, out of ignorance, stubbornness, or both, continued to ring in the New Year on April 1 and were made the butt of jokes and pranks on account of their foolishness. This became an annual tradition, according to this version of events, which ultimately spread throughout Europe.
A major weakness of the calendar-change theory is that it fails to account for an historical record replete with traditions linking this time of year to merriment and tomfoolery dating all the way back to antiquity.