There are several theories on how April Fools’ Day originated, but the most likely is this.
The French used to celebrate New Year’s Day on April 1 with the new spring.
When France adopted the Gregorian calendar in 1564, King Charles IX decreed that New Year’s Day would henceforth be celebrated on January 1.
Not everybody got the word, and even among those who knew of the changes, not everyone wanted to go along with it.
As a result, that year a number of people continued to celebrate New Year’s Day on April 1 as usual, throwing parties and exchanging gifts.
To make fun of the clueless and conservative, various wits marked the non-holiday by mocking those who still celebrated.
They invited people to nonexistent events or gave them absurd presents, and anyone who fell for the joke was dubbed a Poisson d’Avril (“April fish”).
Even emperors were not immune to the April foolishness, centuries later, when Napoleon I got married on April 1, 1810, he was thereafter known as the “April Fish.”
Even after everybody got used to the new calendar, the April fooling continued every year on April 1.
Over the centuries, the holiday spread to England and eventually to America.