April 1st is commonly celebrated as April Fools' Day in Europe, the United States., Australia and Brazil. People often play pranks and practical jokes on each other on this day.
The true origin of April Fools' Day is uncertain, but the commonly accepted explanation dates from 1564 in France. During that year, King Charles IX decreed that France would switch from the Julian calendar to the Gregorian calendar. New Year's Day on the Julian calendar was April 1st, and those who continued to celebrate on that day became known as April's Fools. As time progressed, April Fools' Day spread to other countries.