April Fool's Day: April 1
English and French
To coincide with the vernal equinox and other agriculturally significant events, New Year's Day used to be celebrated in many societies around the first of April. Although Pope Gregory introduced a new calendar for the Christian world in 1562--with the new year beginning on January 1--there were some people who did not hear of the change (or refused to believe it) until much later and continued to celebrate New Year's Day on April 1. Others called these traditionalists "April fools" and played tricks on them, sending them on "fool's errands" or trying to make them believe that something false was true.
In France today, April first is called Poisson d'Avril. French children fool their friends by taping a paper fish to their friends' backs; when the "young fools" discover this trick, the prankster yells "Poisson d'Avril!" (April Fish!) Even though April Fool's Day is not an official American holiday, it is still recognized by many. Most of us are "taken in" at some time or another on April 1.