Before 1562, the Christian calendar began each year on April 1.

In 1562, Pope Gregory introduced a new calendar for the Christian world, and the new year starting from that date began on 1st January.

Nevertheless, some people, who hadn't heard about the change in the date, or didn't believe in it, continued to celebrate New Year's Day on 1st April.

Others took advantage of this and played tricks on them calling them "April fools." They used to send them on a "fool's errand" or make them believe that something false was true.

Today, 1st April is called in France "Poisson d'Avril." In this day, French children fool their friends by taping a paper fish to their friends' backs. When they discover the trick, pranksters start to yell "Poisson d'Avril!" (April Fish!).

Elsewhere, it is also called All Fools' Day.