Here is a final word according to Wikipedia (if you believe everything you read there):
Traditionally, in some countries such as New Zealand, the UK, Australia, Canada, and South Africa, the jokes only last until noon, and someone who plays a trick after noon is called an "April Fool". It is for this reason that newspapers in the U.K. that run a front page April fool only do so on the first (morning) edition. Elsewhere, such as in France, Ireland, Italy, South Korea, Japan, Russia, The Netherlands, Germany, Brazil and the U.S., the jokes last all day.
An optimist would say "lets be multicultural about this". A pessimist would say "I guess we know where our calendar comes from".