April Fools' Day or April Fool's Day (sometimes called All Fools' Day) is an annual celebration on April 1, commemorated by practical jokes and hoaxes. The player(s) of the joke(s) or hoax(es) often exposes their action by shouting "April fool(s)" at the recipient(s). The recipients of these actions are called April fools. Mass media can be involved in these pranks that the following day are reported as such. Although popular since the 19th century, the day is not a public holiday in any country.

Aside from April Fools' Day, the custom of setting aside a day for the playing of harmless pranks upon one's neighbour has historically been relatively common in the world.^[1]

A disputed association between April 1 and foolishness is in Geoffrey Chaucer's The Canterbury Tales (1392).^[2] In the "Nun's Priest's Tale", a vain cock Chauntecleer is tricked by a fox on Syn March bigan thritty dayes and two.^[3] Readers apparently understood this line to mean "32 March", i.e. April 1.^[4] However, it is not clear that Chaucer was referencing April 1, since the text of the "Nun's Priest's Tale" also states that the story takes place on the day when the sun is in the signe of Taurus had y-runne Twenty degrees and one, which cannot be April 1. Modern scholars believe that there is a copying error in the extant manuscripts and that Chaucer actually wrote, Syn March was gon.^[5] If so, the passage would have originally meant 32 days after March, i.e. 2 May,^[6] the anniversary of the engagement of King Richard II of England to Anne of Bohemia, which took place in 1381.

In 1508, French poet Eloy d'Amerval referred to a poisson d’avril (April fool, literally "Fish of April"), possibly the first reference to the celebration in France.^[7] Some writers suggest that April Fools' originated because in the Middle Ages, New Year's Day was celebrated on March 25 in most European towns,^[8] through a holiday that in some areas of France, specifically, ended on April 1,^[9][10] and those who celebrated New Year's Eve on January 1 made fun of those who celebrated on other dates by the invention of April Fools' Day.^[9] The use of January 1 as New Year's Day became common in France only by the mid-16th century,^[6] and the date was not adopted officially until 1564, thanks to the Edict of Roussillon.

In 1561, Flemish poet Eduard de Dene wrote of a nobleman who sent his servants on foolish errands on April 1.^[6]

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 1686, John Aubrey referred to the celebration as "Fooles holy day", the first British reference.^[6] On April 1, 1698, several people were tricked into going to the Tower of London to "see the Lions washed".^[6]

Although no Biblical scholar or historian are known to have mentioned a relationship, some have expressed the belief that the origins of April Fool's Day may go back to the Genesis flood narrative. In a 1908 edition of the Harper's Weekly cartoonist Bertha R. McDonald wrote:

In the UK, an April Fool prank 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 pranks.^[13] Ergo, a person playing a prank after midday is considered the "April fool" themselves.^[14]

In Scotland, April Fools' Day was traditionally called 'Huntigowk Day',^[12] although this name has fallen into disuse. The name is a corruption of 'Hunt the Gowk', "gowk" being Scots for a cuckoo or a foolish person; alternative terms in Gaelic would be Là na Gocaireachd, 'gowking day', or Là Ruith na Cuthaige, 'the day of running the cuckoo'. The traditional prank is to ask someone to deliver a sealed message that supposedly requests 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 next person with an identical message, with the same result.^[12]

In England a "fool" is known by different names around the country, including a "noodle", "gob", "gobby" or "noddy".^[15]

In Ireland, it was traditional to entrust the victim with an "important letter" to be given to a named person. That person would then ask the victim to take it to someone else, and so on. The letter when finally opened contained the words "send the fool further".^[16]

In Poland, prima aprilis ("1 April" in Latin) as a day of pranks is a centuries-long tradition. It is a day when many pranks are played; hoaxes – sometimes very sophisticated – are prepared by people, media (which often cooperate to make the "information" more credible) and even public institutions. Serious activities are usually avoided, and generally every word said on April 1 can be untrue. The conviction for this is so strong that the Polish anti-Turkish alliance with Leopold I signed on April 1, 1683, was backdated to March 31.^[17] However, for some in Poland prima aprilis ends at noon of April 1, and prima aprilis jokes after that hour are considered inappropriate and not classy.

Danes, Finns, Icelanders, Norwegians and Swedes celebrate April Fools' Day (aprilsnar in Danish; aprillipäivä in Finnish). Most news media outlets will publish exactly one false story on April 1; for newspapers this will typically be a first-page article but not the top headline.^[18]

In Italy, France, Belgium and French-speaking areas of Switzerland and Canada, April 1 tradition is often known as "April fish" (poissons d'avril in French, april vis in Dutch or pesce d'aprile in Italian). This includes attempting to attach a paper fish to the victim's back without being noticed. Such fish feature is prominently present on many late 19th- to early 20th-century French April Fools' Day postcards. Many newspapers also spread a false story on April Fish Day, and a subtle reference to a fish is sometimes given as a clue to the fact that it is an April fools' prank.

In Lebanon, an April Fool prank is revealed by saying "كذبة أول نيسان" (which means "April First Lie") at the recipient.

In many Spanish-speaking countries (and the Philippines), "Dia de los Santos Inocentes" (Holy Innocents Day) is a festivity which is very similar to the April Fools' Day, but it is celebrated in late December (27, 28 or 29 depending on the location, or January 10th for East Syrians).

As a Western country, Israel has adopted the custom of pranking on April Fools' Day.^[19]

As well as people playing pranks on one another on April Fools' Day, elaborate pranks have appeared on radio and TV stations, newspapers, websites, and have been performed by large corporations. In one famous prank from 1957, the BBC broadcast a film in their Panorama current affairs series purporting to show Swiss farmers picking freshly-grown spaghetti, in what they called the Swiss Spaghetti Harvest. The BBC were later flooded with requests to purchase a spaghetti plant, forcing them to declare the film a hoax on the news the next day.^[20]

With the advent of the Internet and readily available global news services, April Fools' pranks can catch and embarrass a wider audience than ever before.^[21]

December 28, the equivalent day in Spain, Hispanic America and the Philippines, is also the Christian day of celebration of the "Day of the Holy Innocents". The Christian celebration is a holiday in its own right, a religious one, but the tradition of pranks is not, though the latter is observed yearly. After a prank is played, the cry is made, in some regions of Hispanic America: Inocente palomita que te dejaste engañar ("You innocent little dove that let yourself be fooled"), not to be confused with the second translation of palomita, which is popcorn.

In Mexico, the phrase is ¡Inocente para siempre! which means "Innocent forever!".

In Argentina, the prankster says ¡Que la inocencia te valga!, which roughly translates as a piece of advice on not to be as gullible as the victim of the prank. In Spain, it is common to say just ¡Inocente! (which in Spanish can mean "Innocent!", but also "Gullible!").^[22]

In Colombia, the term used is "Pásala por Inocentes", which roughly means: "Let it go; today it's Innocent's Day."

In Belgium, this day is also known as the "Day of the innocent children" or "Day of the stupid children". It used to be a day where parents, grandparents, and teachers would fool the children in some way. But the celebration of this day has died out in favor of April Fools' Day.

Nevertheless, on the Spanish island of Menorca, Dia d'enganyar ("Fooling day") is celebrated on April 1 because Menorca was a British possession during part of the 18th century. In Brazil, the "Dia da mentira" ("Day of the lie") is also celebrated on April 1.^[22]

The practice of April Fool pranks and hoaxes is controversial.^[14][23] The mixed opinions of critics are epitomized in the reception to the 1957 BBC "Spaghetti-tree hoax", in reference to which, newspapers were split over whether it was "a great joke or a terrible hoax on the public".^[24]

The positive view is that April Fools' can be good for one's health because it encourages "jokes, hoaxes...pranks, [and] belly laughs", and brings all the benefits of laughter including stress relief and reducing strain on the heart.^[25] There are many "best of" April Fools' Day lists that are compiled in order to showcase the best examples of how the day is celebrated.^[26] Various April Fools' campaigns have been praised for their innovation, creativity, writing, and general effort.^[27]

The negative view describes April Fools' hoaxes as "creepy and manipulative", "rude" and "a little bit nasty", as well as based on schadenfreude and deceit.^[23] When genuine news or a genuine important order or warning is issued on April Fools' Day, there is risk that it will be misinterpreted as a joke and ignored – for example, when Google, known to play elaborate April Fools' Day hoaxes, announced the launch of Gmail with 1-gigabyte inboxes in 2004, an era when competing webmail services offered 4-megabytes or less, many dismissed it as a joke outright.^[28][29] On the other hand, sometimes stories intended as jokes are taken seriously. Either way, there can be adverse effects, such as confusion,^[30] misinformation, waste of resources (especially when the hoax concerns people in danger) and even legal or commercial consequences.^[31][32]

People obeying hoax messages to telephone "Mr.C.Lion" and "Mr.L.E.Fant" and suchlike at a telephone number that turns out to be a zoo, sometimes cause a serious overload to zoos' telephone switchboards.

Other examples of genuine news on April 1 mistaken as a hoax include:

Books, films, telemovies and television episodes have used April Fool's Day as their title or inspiration. Examples include Bryce Courtenay's novel April Fool's Day (1993), whose title refers to the day Courtenay's son died. The 1990s sitcom Roseanne featured an episode titled "April Fools' Day". This turned out to be intentionally misleading, as the episode was about Tax Day in the United States on April 15 – the last day to submit the previous year's tax information.