The history of April Fool's Day or All Fool's Day is uncertain, but the current thinking is that it began around 1582 in France with the reform of the calendar under Charles IX. The Gregorian Calendar was introduced, and New Year's Day was moved from March 25 - April 1 (new year's week) to January 1st.
Communication traveled slowly in those days and some people were only informed of the change several years later. Still others, who were more rebellious refused to acknowledge the change and continued to celebrate on the last day of the former celebration, April 1st. These people were labeled "fools" by the general populace, were subject to ridicule and sent on "fool errands," sent invitations to nonexistent parties and had other practical jokes played upon them. The butts of these pranks became known as a "poisson d'avril" or "April fish" because a young naive fish is easily caught.
Now that you have all that information - hope you have a fun day!!