Why is April 1 called April fools' day?

In the UK, an April Fool joke is revealed by shouting "April fool!" at the recipient, who becomes the "April fool".

New Delhi, Apr 1: April 1, sometimes called as April Fools' Day,or All Fools' Day, is a day when many take the liberty to play pranks at others. It is a day aside a day for the playing of harmless pranks on others.

Some see it as a celebration related to the turn of the seasons, while others believe it stems from the adoption of a new calendar. Ancient cultures, including those of the Romans and Hindus, celebrated New Year's Day on or around April 1. It closely follows the vernal equinox (March 20th or March 21st.) In medieval times, much of Europe celebrated March 25, the Feast of Annunciation, as the beginning of the new year, according to infoplease.com.

Starting as a custom of sending people on false errands, April Fools' Day seems to have migrated to England from France in the late 1600s. The true origins of the holiday remain unknown.

One popular legend is that April Fools' Day began with France's 1564 Edict of Roussillon, which decreed that New Year's Day be moved to January 1st. Depending on which part of France someone lived in back then, they might've celebrated the new year on Easter. Those who continued to celebrate the old New Year were called "April fools" by the January 1 early adopters.

A disputed association between April 1 and foolishness is in Geoffrey Chaucer's The Canterbury Tales (1392). In the "Nun's Priest's Tale", a vain cock Chauntecleer is tricked by a fox on Syn March bigan thritty dayes and two Readers apparently understood this line to mean "32 March", i.e. April 1. However, it is not clear that Chaucer was referencing April 1.

In the Netherlands, the origin of April Fools' Day is often attributed to the Dutch victory at Brielle in 1572, where the Spanish Duke Álvarez de Toledo was defeated. "Op 1 april verloor Alva zijn bril" is a Dutch proverb, which can be translated to: "On the first of April, Alva lost his glasses." In this case, the glasses ("bril" in Dutch) serve as a metaphor for Brielle. This theory, however, provides no explanation for the international celebration of April Fools' Day.

In the UK, an April Fool joke is revealed by shouting "April fool!" at the recipient, 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.[12] This continues to be the current practice with the holiday ceasing at noon, after which time it is no longer acceptable to play jokes. A person playing a joke after midday is considered the "April fool" themselves.