April first: pesce d'aprile!

Il primo d'aprile è una tradizione presente in molti paesi del mondo, seppur con nomi e aspetti diversi.
In Inghilterra e negli Stati Uniti si usa l'espressione April fools’ day (il giorno degli sciocchi d'aprile).

April Fools' Day is celebrated in different countries around the world on April first every year. Sometimes referred to as All Fools' Day, April 1st is not a national holiday, but is widely recognized and celebrated as a day when many people play all kinds of jokes and foolishness. The day is marked by the commission of good-humoured or otherwise funny jokes, hoaxes, and other practical jokes of varying sophistication on friends, family members, teachers, neighbors, work associates, etc.
Traditionally, in some countries such as New Zealand, Ireland, the UK, Australia, and South Africa, the jokes only last until noon, and someone who plays a trick after noon is called an "April Fool" and taunted "April Fool's Day's past and gone, You're the fool for making one." Elsewhere, such as in France, Italy, South Korea, Japan, Russia, The Netherlands, Germany, Brazil, Canada, and the U.S., the jokes last all day.
(fonte: Wikipedia)

Per saperne di più:
http://pieravincenti.wordpress.com/2011/04/01/pesce-daprile-origini-e-tradizioni-nel-mondo/

Un simpatico esempio di scherzo:
http://mail.google.com/mail/help/motion.html