April Fools’ Day (sometimes called All Fools’ Day) is an informal holiday celebrated every year on April 1. The day is not a national holiday in any country, but it is widely recognized and celebrated as a day when people play practical jokes and hoaxes on each other, called April fools. Hoax stories are also often found in the press and media on this day.
Early “April Fools Day” may have included the Roman Festival of Hilaria, held March 25th, and the Medieval Feast of Fools, held December 28, still a day where pranks are played in Spanish-speaking countries. As early as the 1300’s scholars and writers were writing and referring to pranks and trickery.
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 theEdict of Roussillon. 
Customs From Other Countries
In the U.K., an April Fool joke is revealed by shouting “April Fool” at the recipient. In Australia the joking ceases at midday and the person playing a joke in the afternoon is then the “April Fool” themselves.
In Iran, jokes are played on the 13th day of the Persian New Year (Nowruz), which falls on April 1 or April 2 and has been celebrated as far back as 536 BC. It’s called Sizdah Bedar and is the oldest prank tradition in the world today. Many believe that April Fools Day originated in this tradition.
If you have been pranked before or are the prankster just know that this tradition has been occurring for centuries and looks to continue for many years to come. So mark your calendars and keep alert because April Fools Day looks to be here for a very long time!
 Source Wikipedia