The most popular explanation is the switch to the Gregorian calendar in 1582. Prior to that, the new year was celebrated for eight days, from March 25 until April 1. Those who did not adapt to the new calendar system, or did not learn about it, continued to celebrate New Year’s Day on April 1. People began making fun of these traditionalists, sending them on “fool’s errands” or trying to trick them into believing something false.
These April “fools” were subjected to ridicule and practical jokes and the tradition to do practical jokes on April 1 was born.