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April Fools' Day, 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.

Ancient cultures, including those of the Romans and Hindus, celebrated New Year's Day on or around April 1. It closely follows the vernal equinox (March 20th or March 21st.) In medieval times, much of Europe celebrated March 25, the Feast of Annunciation, as the beginning of the new year.

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. Other people began to make fun of these traditionalists, sending them on "fool's errands" or trying to trick them into believing something false. Eventually, the practice spread throughout Europe.

April Fools' Day is observed throughout the Western world. Practices include sending someone on a "fool's errand," looking for things that don't exist; playing pranks; and trying to get people to believe ridiculous things.

The French call April 1 Poisson d'Avril, or "April Fish." French children sometimes tape a picture of a fish on the back of their schoolmates, crying "Poisson d'Avril" when the prank is discovered.