Happy New Year, Fool!
I'm such a cynic - I was sure a holiday as inane as April Fool's Day must have been "invented" by Hallmark. As it happens, I was wrong.

In the late 16th century, Pope Gregory XIII replaced the less accurate Julian calendar, in use since 45 B.C., with his Gregorian calendar. This calendar began the New Year on January 1st, as opposed to April 1st. Well, our French friends didn't like this one bit, and continued to celebrate on the first of April. They were widely ridiculed and sent on "fools' errands". In Great Britain and North America, April 1st became April Fool's Day, to further mock these French traditionalists. So much for papal bull.