April Fool's Day is on April the first, a day when people play tricks on others, especially by telling them something that is not true. Its exact origin is uncertain, but the possible origin may be from France. In France, the celebration of the new year used to begin on March 25th and finish on April 1st. In 1582, a new calendar was introduced (the Gregorian Calendar), and New Year's Day was changed to January 1st. For many years, however, many people still celebrated New Year's Day on April 1st instead of on January 1st. They did not know about the new calendar, so they were called "fools" by the people who followed the new modern calendar.