There are several explanations for the origin of April Fools' Day, but here is the most plausible one. April 1st was once New Year's Day in France. In 1582, Pope Gregory declared the adoption of his Gregorian calendar to replace the Julian calendar and New Year's Day was officially changed to January 1st. It took awhile for everyone in France to hear the news of this major change and others obstinately refused to accept the new calendar, so a lot of people continued to celebrate New Year's Day on the first of April – earning them the name April fools. The April fools were subjected to ridicule and practical jokes and the tradition was born. The butts of these pranks were first called poisson d'avril or April fish because a young naive fish is easily caught. A common practice was to hook a paper fish on the back of someone as a joke. This evolved over time and a custom of prank-playing continues on the first day of April.
--Taken from Dictionary.com.
