April Fools’ Day (alternatively April Fool’s Day, sometimes All Fools’ Day) is celebrated on April 1 every year. April 1 is not a national holiday, but is widely recognized and celebrated in various countries as a day when people play practical jokes and hoaxes on each other called April fools.
In Italy, France and Belgium, children and adults traditionally tack paper fishes on each other’s back as a trick and shout “April fish!” in their local languages (pesce d’aprile!, poisson d’avril! and aprilvis! in Italian, French and Flemish, respectively). Such fish feature prominently on many late 19th to early 20th century French April Fools’ Day postcards.
The earliest recorded association between April 1 and foolishness is an ambiguous reference in Chaucer‘s Canterbury Tales (1392). Many writers suggest that the restoration of January 1 by Pope Gregory XIII as New Year’s Day of the Gregorian Calendar in the 16th century was responsible for the creation of the holiday, sometimes questioned for earlier references.
Precursors of April Fools’ Day include the Roman festival of Hilaria, held March 25, and the Medieval Feast of Fools, held December 28, still a day on which pranks are played in Spanish-speaking countries.
In Chaucer’s Canterbury Tales (1392), the “Nun’s Priest’s Tale” is set Syn March bigan thritty dayes and two.  Modern scholars believe that there is a copying error in the extant manuscripts and that Chaucer actually wrote, Syn March was gon. Thus, the passage originally meant 32 days after April, i.e. May 2, the anniversary of the engagement of King Richard II of England to Anne of Bohemia, which took place in 1381. Readers apparently misunderstood this line to mean “March 32”, i.e. April 1. In Chaucer’s tale, the vain cock Chauntecleer is tricked by a fox.
In 1508, French poet Eloy d’Amerval referred to a poisson d’avril (April fool, literally “April fish”), a possible reference to the holiday. In 1539, Flemish poet Eduard de Dene wrote of a nobleman who sent his servants on foolish errands on April 1. In 1686, John Aubrey referred to the holiday as “Fooles holy day”, the first British reference. On April 1, 1698, several people were tricked into going to the Tower of London to “see the Lions washed”.
In the Middle Ages, up until the late 18th century, New Year’s Day was celebrated on March 25 (Feast of the Annunciation) in most European towns. In some areas of France, New Year’s was a week-long holiday ending on April 1. Many writers suggest that April Fools originated because those who celebrated on January 1 made fun of those who celebrated on other dates. The use of January 1 as New Year’s Day was common in France by the mid-16th century, and this date was adopted officially in 1564 by the Edict of Roussillon.
In the UK, an April fool joke is revealed by shouting “April fool!” at the victim, who becomes the “April fool”. A study in the 1950s, by folklorists Iona and Peter Opie, found that in the UK, and in countries whose traditions derived from the UK, the joking ceased at midday. A person playing a joke after midday is the “April fool” themselves. But this practice appears to have lapsed in more recent years.
In Scotland, April Fools’ Day is traditionally called Hunt-the-Gowk Day (“gowk” is Scots for a cuckoo or a foolish person), although this name has fallen into disuse. The traditional prank is to ask someone to deliver a sealed message requesting help of some sort. In fact, the message reads “Dinna laugh, dinna smile. Hunt the gowk another mile”. The recipient, upon reading it, will explain he can only help if he first contacts another person, and sends the victim to this person with an identical message, with the same result.
In Iran, jokes are played on the 13th day of the Persian new year (Nowruz), which falls on April 1 or April 2. This day, celebrated as far back as 536 BC, is called Sizdah Bedar and is the oldest prank-tradition in the world still alive today; this fact has led many to believe that April Fools’ Day has its origins in this tradition.
The April 1 tradition in France, Romandy and French-speaking Canada includes poisson d’avril (literally “April’s fish”), attempting to attach a paper fish to the victim’s back without being noticed. This is also widespread in other nations, such as Italy, where the term Pesce d’aprile (literally “April’s fish”) is also used to refer to any jokes done during the day. This custom also exists in certain areas of Belgium, including the province of Antwerp. The Flemish tradition is for children to lock out their parents or teachers, only letting them in if they promise to bring treats the same evening or the next day.
In Poland, prima aprilis (“April 1” in Latin) is a day full of jokes; various hoaxes are prepared by people, media (which sometimes cooperate to make the “information” more credible) and even public institutions. Serious activities are usually avoided. This conviction is so strong that the anti-Turkish alliance with Leopold I signed on April 1, 1683, was backdated to March 31.
All info from http://en.wikipedia.org/wiki/April_Fools%27_Day