From Wilstar Greetings:
Unlike most of the other nonfoolish holidays, the history of April Fool’s Day, sometimes called All Fool’s Day, is not totally clear. There really wasn’t a “first April Fool’s Day” that can be pinpointed on the calendar. Some believe it sort of evolved simultaneously in several cultures at the same time, from celebrations involving the first day of spring.
It’s better to keep your mouth shut and be thought a fool than to open it and leave no doubt. –Mark Twain