April Fools' Day, celebrated on April 1st, is a day dedicated to practical jokes, hoaxes, and good-natured pranks, with origins that remain somewhat mysterious.

Some believe it comes from a Chaucer tale relating to March 32nd - but there are other French suggestions of the origins

The Wiki bit..

Although many theories have been proposed throughout the years, the origin of April Fools' Day is not exactly known.

In Geoffrey Chaucer's The Canterbury Tales (1392).[3] In the "Nun's Priest's Tale", a vain cock, Chauntecleer, is tricked by a fox "Since March began, full thirty days and two,"[4][5]i.e. the 32nd day from 1 March, which is 1 April.[6] However, it is not clear that Chaucer was referencing 1 April 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 sign of Taurus had y-rune Twenty degrees and one," which would not be 1 April. Modern scholars believe that there is a copying error in the extant manuscripts and that Chaucer actually wrote, "Syn March was gon".[7] If so, the passage would have originally meant 32 days after March ended, i.e. 2 May.[8]

In 1508, French poet Eloy d'Amerval referred to a poisson d'avril (April fool, literally "April's fish"), possibly the first reference to the celebration in France.[9] Some historians suggest that April Fools' originated because, in the Middle Ages, New Year's Daywas celebrated on 25 March in most European towns,[10] with a holiday that in some areas of France, specifically, ended on 1 April,[11][12] and those who celebrated New Year's Day on 1 January made fun of those who celebrated on other dates by the invention of April Fools' Day.

The use of 1 January as New Year's Day became common in France only in the mid-16th century. The poet Eduard de Dene of a nobleman sent his servant on foolish errands on 1 April, predating the change.[8] April Fools' Day was also an established tradition in Great Britain before 1 January was established as the start of the calendar year.[15][16]

An 1857 ticket to "Washing the Lions" at the Tower of London. No such event ever took place.

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