I often wonder how things got started. Today is an example. Who thought of April Fool's Day? Why? When? Etc., etc.
So, of course, I snopes-ed. Perhaps it has something to do with changing to a Gregorian calendar (and you thought they were just into chanting). Or it may have something to do with the Vernal Equinox, which sounds like an undesirable disease to me.
Whatever the origin, we've taken it upon ourselves on days like today to play jokes on those closest to us. So, enjoy your day and watch out for others enjoying theirs.