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April: April Fish
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The commonly accepted origin of April Fool's Day involves changes in the calendar. At one time, the New Year celebration began on March 25 and ended on April 1. However, in 1582, King Charles IX adopted the Gregorian calendar and accepted the beginning of the new year as January 1. Those who refused to acknowledge the new date or simply forgot received foolish gifts and invitations to nonexistent parties. The butt of such a prank was known as a "poisson d'avril" or "April fish."
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[F]or years after the king's command, many people still celebrated New Year's Day on April 1. The more in-the-know French folk poked fun at their out-of-date friends. The old-fashioned French were considered simple-minded, called "fools," and nicknamed "poisson d'Avril," - in English, "April fish" - named after an easily-caught native fish. April fish were teased and invited to non-existent New Year's Day parties on April 1. Paper fish were affixed to their backs.
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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.
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