Here is a great explanation of April 1st that I got from www.care2.com:
For those of you who live in a place where April Fool's Day is not celebrated, you're probably wondering what all this silliness is about... April Fool's Day is indeed a silly day. It is a time when it is perfectly acceptable to play pranks, jokes and surprises on everyone you know.
The origins are a bit unclear, but it seems that the fun may have started in 1582 in France. In that year, King Charles IX decreed that France would switch to the Gregorian calendar, thereby moving "New Year's Day" from March 25 to January 1. Those people that forgot about, or chose to ignore, the date change were sent foolish gifts and invited to imaginary parties. The butt of these jokes was called a "poisson d'avril" or "April Fish".