Which is it? April Fools' Day or April Fool's Day. Grammars.com tells me that it is plural possessive, and that the singular is a variant spelling. I figured that it must be plural. However, the usage is split evenly for each spelling.
What is the earliest reference? There is a dispute over Geoffrey Chaucer's The Canterbury Tales (1392) reference to "Syn March bigan thritty dayes and two." The discussion is whether Chaucer meant this to be April 1st or whether it was a copying error.
In 1508, French poet Eloy d'Amerval referred to a "poisson d'avril." There we have it - the expression used in France and other European countries. In the past, the idea was to attach a paper fish to the victim's back without being noticed.
In the U.K. the holiday 'ceases' at noon, according to Wikipedia. One can be called a 'fool', 'noodle', 'gob', 'gobby' or 'noddy' in England. That's a lot of choices. Other countries consider April Fools' jokes to finish by noon. After that they are considered inappropriate and not classy.
Here's a preview of how outrageous April Fools' jokes have been:
"I never did anything wrong, and I won't do it again," said former President Richard Nixon, announcing that he would run for president in 1992."
National Public Radio’s piece on Nixon’s 1992 presidential run is one of its most famous April Fools’ Day pranks. Not only did people believe it, they were outraged.