Though pranksters and joke-lovers in many countries now gleefully prepare to dupe friends and loved ones on April Fool's Day, no one knows exactly when or why, or even where, this tradition began.

A giddy spurt of practical joking seems to have coincided with the coming of spring since the time of the Ancient Romans and Celts, who celebrated a festival of mischief-making. The first mentions of an All Fool's Day (as it was formerly called) came in Europe in the Middle Ages.

Some trace April Fool's Day back to Roman mythology, particularly the story of Ceres, Goddess of the harvest, and her daughter, Proserpina.

Pluto, God of the Dead, abducted Proserpina and took her to live with him in the underworld. The girl called out to her mother, but Ceres could only hear the echo of her daughter's voice and searched for her in vain.

The most widespread theory of the origin of April Fool's Day is the switch from the old Julian to the Gregorian calendar (now in use) in the late 16th century. Under the Julian calendar, the New Year was celebrated during the week between March 25 and April 1, but under the Gregorian calendar, it was moved to Jan. 1. Those who were not notified of the change, or stubbornly kept to the old tradition, were often mocked and had jokes played on them on or around the old New Year.

In France, this took the form of pranksters sticking fish on the backs of those who celebrated the old custom, earning the victims of the prank the name Poisson d'Avril, or April Fish.