Sometimes called All Fools' Day, is one of the most light-hearted days of the year. Its origins are uncertain. Some see it as a celebration related to the turn of the seasons, while others believe it stems from the adoption of a new calendar. Every year it is celebrated on 1st April.
In 1582, Pope Gregory XIII ordered a new calendar (the Gregorian Calendar) to replace the old Julian Calendar. The new calendar called for New Year's Day to be celebrated Jan. 1. That year, France adopted the reformed calendar and shifted New Year's day to Jan. 1. According to a popular explanation, many people either refused to accept the new date, or did not learn about it, and continued to celebrate New Year's Day on April 1.
The "fools' errands" we play on people are practical jokes. Putting salt in the sugar bowl for the next person is not a nice trick to play on a stranger. College students set their clocks an hour behind, so their roommates show up to the wrong class - or not at all. Some practical jokes are kept up the whole day before the victim realizes what day it is. Most April fool jokes are in good fun and not meant to harm anyone.
The April 1 tradition in France, Romandy and French-speaking Canada includes poisson d'avril (literally "April's fish"), attempting to attach a paper fish to the victim's back without being noticed. In Poland, prima aprilis ("April 1" in Latin) is a day full of jokes; various hoaxes are prepared by people, media (which sometimes cooperate to make the "information" more credible) and even public institutions. In Denmark, May 1 is known as "Maj-kat", meaning "May-cat", and is also a joking day.