How did April fools start?

Nobody knows for sure how April Fools Day started, but some big possibilities are the adaption of a new calendar in the year of 1582, some say that it even traces back to 1392 where in a play/story called 'The Nun's Priest's Tale' where a fox tricks a rooster. Some say it was a celebration of Spring coming, and in the past many of the spring celebrations where held in the beginning of April - These are the main ones.