In Britain, fooling at this time of year has gone on for centuries, however the origin of the custom still remains obscure. There are several theories to account for it.
One theory is that in Europe, until the sixteenth century, March 25th, the first day of the Vernal Equinox Festival, was New Year's Day. On April 1st, the last day of the festival, people used to give presents to one another. In 1564, Charles IX, the French king, adopting the Gregorian calendar and fixed January 1st as New Year's Day. Those who were against the revision continued to express their complaints by giving presents or paying New Year's visits on April 1st. In the following years, these traditionalists who insisted on celebrating the New Year at its old time were mocked as fools and people would play pranks and tricks on them and called them ` Poisson d'avril ', meaning April Fish. This must have been so much fun that it spread all over the world and people played tricks on everyone, not just the people who didn't accept the new calendar.