The official roots of April Fool's Day are a mystery. However, some
theories date it back to 1582 in France, when the transition occurred between use of the Julian Calendar (Where the last week of March through April 1st was considered the beginning of the calendar year) and the Gregorian Calendar (currently used). People who continued to celebrate the New Year in March/April were considered "fools" and worthy of pranking due to their foolishness.
It's interesting to note that during the use of the Julian Calendar, different cultures chose to "begin" the year at several different times, ranging from October through March, aligning it with local festivals, the harvest, the spring equinox, or religious celebrations.