An internet search for why April 1st is celebrated as a Fool’s Day came up with several theories. One theory is that back in 1562, Pope Gregory XIII introduced a new calendar that changed the date of the New Year. Those that continued to use the old calendar were dubbed April Fools. A second theory, falls along the same lines but states that Charles IX in the year 1564 introduced the Gregorian Calendar with the same results. There appears to be no conclusive conclusion as to why and it remains open for debate.