April Fools' Days is celebrated annually with practical jokes and trickery on 1 April, with even national newspapers publishing false reports to catch out unsuspecting readers. But where did the tradition come from – and why do we celebrate it?

One of the most widely accepted theories of the origin of April Fools' Day relates to the European transition from the Julian calendar to the Gregorian. In 1563, Pope Gregory XIII issued a papal bull decreeing that Christian countries should adopt a new, standardised calendar. Consequently, New Year's Day moved from 1 April to the 1 January.

Announcing news of the transition among rural populations was a difficult task, particularly following France's switch to the new calendar in 1582. People who continued to celebrate the New Year at the end of March were targeted with jokes and tricks – which led to April Fools' Day.

Literary scholars have suggested that Geoffrey Chaucer's The Nun's Priest's Tale, written in the 1390s, is one of the earliest references to April Fools' Day with the story of a vain cock called Chauntecleer who falls for the tricks of a fox.

The day is also cited in 16th and 17th century poetry. In 1508, the French poet Eloy d'Amerval wrote of a "poisson d'avril" – which translates as an "April fish", or April fool. The Flemish poet Eduard de Dene wrote about a nobleman who sent his servants on foolish errands on the day in 1539, while John Aubrey made the first British reference to the "Fooles holy day" in 1686.

In Chaucer\'s Nun\'s Priest\'s Tale, the vain cock Chauntecleer is tricked by a fox Getty

How is it celebrated around the world?

In Scotland, April Fools' Day is traditionally called Hunt-the-Gowk Day, the term "gowk" refers to a cuckoo or a foolish person. The traditional prank is to ask someone to deliver a sealed message requesting some sort of help. The message actually reads: "Dinna laugh, dinna smile. Hunt the gowk another mile" – to which the recipient will explain he can only help if he first contacts another person and the message is sent on.

In France and French-speaking areas of Canada, a paper fish is attached to the victim's back, known as the poisson d'Avril. The same tradition is carried out in Italy, as the Pesce d'aprile.

The Flemish tradition on April Fools' Day is for children to lock their parents or teachers out of the house or classroom, only letting them in if they promise to bring treats. In Iran, April Fools' Day takes place on the 13th day of the Persian new year, called Sizdah Be-dar, which falls on 1 April.

It is tradition to only play jokes on people until midday on April 1 – after which the joker who plays the prank becomes the fool.