How did April Fool’s Day originate?

The history of April Fool's Day  is not totally clear. Most people identify the beginning of April Fool's Day to be 1582, in France. Prior to that year, the New Year was celebrated for eight days, beginning on March 25. The celebration culminated on April 1. When the Gregorian Calendar was introduced, New Year's Day was moved to January 1.

Since the news traveled by foot, many people did not receive this news for several years. Others refused to accept the new calendar and continued to celebrate the New Year on April 1. These folk were labeled "fools" by the public. They were subject to some ridicule, and were often sent on "fools errands" or were made the butt of practical jokes. This harassment evolved, over time, into a tradition of prank-playing on the first day of April. It spread around the world and April Fools' Day became an international fun fest.

Source: http://southeast.patch.com/articles/april-fools-day-how-did-it-start-554e2518