April 1, the one day licensed for lies, with a founding story that falls apart on inspection.
Documented
The Flemish poem
In 1561 the Flemish poet Eduard De Dene wrote of a nobleman who sends his servant back and forth on absurd errands on April 1. It is the first unambiguous description of the custom in print.
Assessment: Documented: the poem survives and predates the usual explanations.
Stories
The New Year switch
The standard story: when France moved New Year's Day from late March to January 1 under Charles IX in 1564, stubborn celebrants of the old date became April fools. Neat, but the 1561 poem predates the French reform, and the custom spread across countries with different calendars.
Assessment: Stories: tidy, popular, and out of order with the dates.
Interesting claims
Spring misrule festivals
Writers link the day to Rome's Hilaria or India's Holi, spring festivals of inversion and jokes. The resemblance is real; the paper trail connecting them to April 1 is not.
Assessment: Interesting claims: parallel evolution is the likelier answer, which is less fun.
Sources and further reading
Eduard De Dene, Flemish poem (1561): a nobleman sends his servant on fool's errands on April 1
Edict of Roussillon, Charles IX (1564/1567): France's calendar reform behind the popular origin theory
Records of the Roman Hilaria festival, March 25, often cited without a connecting paper trail
Worth asking
The theory everyone repeats about calendar reform is younger than the evidence that undermines it; what else do we all know that was wrong from the start?