The history of April Fools’ Day, or All Fools’ 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 1.
Many people were rebellious and refused to acknowledge the change. They continued to celebrate New Year’s on the last day of the former celebration, April 1.
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.