The origin of April Fools' Day most likely began with King Charles IX of France in 1564 (although some say it started with Pope Gregory XIII in 1582). King Charles IX ordered a new calendar to replace the old Julian calendar, which was then named the Gregorian calendar. The Gregorian calendar changed the order of the months and thus New Year's Day was moved to January 1. Before the change, the New Year was celebrated on March 25, the advent of spring, and ended on April 1.
Historians are not sure if some French simply resisted the change or were not told about it, but they continued to hold their celebrations during the week of April 1. Many people ridiculed them and sent them fake gifts and invitations to nonexistent parties. They were often the butt of jokes around this time and called ''fools'' for celebrating New Year's on April 1 instead of the newly adopted January 1.
Through time, the French adopted April 1 as a new celebration of these events and called it Poisson d'Avril meaning ''April fish.'' The significance of the term ''April fish'' has to do with the zodiac. At that time of year, the sun was leaving the zodiacal sign of Pisces, the fish, and was therefore of significance to the previous New Year celebration.
The customary French tradition for celebrating Poisson d'Avril is for children to make paper fish and run around taping the fish to their friends' backs. In essence, making them a fool. Once the ''victim'' realized what had happened, the prankster would cry out ''Poisson d'Avril'' and the pranks would continue. It appears that this French tradition was such great fun that it reached other countries, including England and then America, although it took nearly 200 years to spread.