Ancient religions celebrated April 1st as the New Year. This is when Spring brought new plantings of seeds for food for all in the Summer. It was a time of great celebration. The Roman Catholics changed the day to a day for ‘fools’.
This is the exact same thing they did with Pagan Sun of God rising on December 25th. This is when the Sun, having died on the Southern Cross constellation for 3 months, began to rise off the cross. This story of the Sun of God rising became the Son of God, Christmas Day.
One theory for its origin, and perhaps the most widely accepted, relates to the European transition from the Julian to the Gregorian calendar. At the Council of Trent in 1563, Pope Gregory XIII issued a papal bull decreeing that Christian countries should adopt a new standardised calendar. Now the internationally accepted calendar in the Western World, the Gregorian calendar was adopted due to its greater accuracy compared to the Julian calendar. One consequence of the transition however, was that New Years Day moved from 1st April to the 1st January.
Although resisted by the Protestant countries in Europe, the new calendar was officially adopted by the Catholic states reasonably quickly. However, presenting the news of the transition across largely rural populations was a much slower task. Supposedly, this was a particular problem following France’s switch to the new calendar in 1582. People who continued to celebrate the New Year at the end of March became the targets for jokes, pranks and hoaxes. This included having paper fish attached to their backs, and being called ‘April fish’, in reference to their supposed gullibility.