The History of April Fools Day...

When King Charles the IX of France introduced the Gregorian calendar in 1582 it changed the first day of the new year to January 1st. Before that it was celebrated during the week of March 25 thru April 1. Those that refused to acknowledge the change continued to celebrate on April 1st and were considered "fools" for not changing. It became commonplace to play jokes on these people.