"April 1st: This is the day upon which we are reminded of what we are on the other three-hundred and sixty-four." — Mark Twain

http://en.wikipedia.org/wiki/April_Fools%27_Day

The April 1 tradition in France includes poisson d'avril (literally "April's fish"), attempting to attach a paper fish to the victim's back without being noticed. This is also widespread in other nations, such as Italy (where the term pesce d'aprile (literally "April's fish") is also used to refer to any jokes done during the day).