The origin of this custom has been much disputed. The most commonly accepted (or at least restated) theory goes like this: Up until 1564, New Year's Day in France was March 25th. But since March 25th so often fell during Holy Week, the Church generally postponed its celebration to April 1st. In that year, France adopted the Gregorian Calendar and New Year's Day was officially changed to January 1st — but many people still called out New Year's greetings to their friends on April 1st. Such persons were therefore "April fools," and a tradition developed that these people were easily tricked in one way or another. Eventually it spread throughout the rest of Europe.

The problem with this theory is that it does not conform with historical records, which show that the first day of April was linked with celebrations as many as 1,500 years earlier.