April Fool's Day is traditionally a day to play practical jokes on others, send people on fool's errands, and fool the unsuspecting. No one knows how this holiday began but it was thought to have originated in France.
The closest point in time that can be identified as the beginning of this tradition was in 1582, in France. New Year's was celebrated on March 25 and celebrations lasted until April 1st. When New Year's Day as changed from March 25 to January 1st in the mid-1560's by King Charles IX, there were some people who still celebrated it on April 1st and those people were called April Fools.
Pranks performed on April Fool's Day range from the simple, (such as saying, "Your shoe's untied!), to the elaborate. Setting a roommate's alarm clock back an hour is a common gag. The news media even gets involved. For instance, a British short film once shown on April Fool's Day was a fairly detailed documentary about "spaghetti farmers" and how they harvest their crop from the spaghetti trees. Whatever the prank, the trickster usually ends it by yelling to his victim, "April Fool!"
April Fool's Day is a "for-fun-only" observance. Nobody is expected to buy gifts or to take their "significant other" out to eat in a fancy restaurant. Nobody gets off work or school. It's simply a fun little holiday, but a holiday on which one must remain forever vigilant, for he may be the next April Fool!
Each country celebrates April Fool's differently. In France, the April Fool's is called "April Fish" (Poisson d'Avril). The French fool their friends by taping a paper fish to their friends' backs and when some discovers this trick, they yell "Poisson d'Avril!".