A start date no one can pin down
One of the earliest written traces of the French expression poisson d’avril (literally “April fish”) appears in Pierre Michault’s Le Doctrinal du temps présent in 1466, and again in Eloy d’Amerval around 1508. Back then the phrase didn’t quite mean “prank.” It described a go-between, the messenger you sent to handle other people’s romantic errands. The modern sense (a joke played on April 1st, complete with a paper fish stuck on someone’s back) only hardens into a stable custom in the 17th century.
In other words: we have been playing the April fool for far longer than we have been able to explain why.
The calendar-shuffle theory
Here is the version everyone repeats. Before the calendar reform, the year began in France on different dates depending on the diocese: Easter, March 25 (the Annunciation), sometimes March 1, sometimes early April. New Year’s gifts and festivities therefore fell at the end of winter.
Then King Charles IX signed the Edict of Roussillon in 1564, which formally fixed the start of the year on January 1. The story goes that it took the kingdom a while to adjust. Those who, out of habit or carelessness, kept exchanging good wishes in April were mocked for their lateness, handed fake gifts and paper fish in return.
It is an elegant story. It has been making the rounds since the 19th century. But historians stay cautious: no contemporary source actually links the royal edict to the prank. The explanation was almost certainly built later, to make sense of a tradition whose real origin had already slipped away. The clincher is chronology: the expression was already on the page in 1466, a full century before the edict, and the Flemish poet Eduard de Dene describes a nobleman sending his servant on absurd errands on April 1 in 1561, three years before Charles IX put quill to parchment.
Other, quieter trails
There are several. April 1 falls just after the astrological sign of Pisces, which could account for the fish. In Christian tradition, Lent winds down around this time, and fish (the food of the fast) becomes a shared image. And more simply, the first warmth of spring calls for lightness. Many cultures place their prank traditions in March or April: April Fools’ across the English-speaking world, the Spanish Día de los Santos Inocentes (December 28, an outlier on the calendar but the same instinct), the Persian Sizdah Bedar.
The likeliest answer is that no single thread carries the whole weight. April 1 is a millefeuille of historical coincidences, with each generation adding its own layer: the joke, the paper fish on the back, the fake newspaper article, and now the fake tweet. The tradition has spread far more reliably than any explanation for it.