April Fool's Day is celebrated annually on April 1st and involves playing pranks on unsuspecting people, calling them "April fools." The tradition originated in 1562 when Pope Gregory introduced a new calendar moving New Year's Day to January 1st, but some were unaware of the change and continued celebrating on April 1st, leading others to play tricks on them. Common April Fool's pranks today include attaching fake fish to people's backs in France and Italy or sending people on "fool's errands" to convince them of something false.