April Fools' Day (or All Fools' Day) is celebrated April 1st, as we all know. The earliest likely written reference is in Geoffrey Chaucer's Canterbury Tales back in 1392. In 1509, a French poet referred to a poisson d’avril, likely a reference to the holiday. John Aubrey's 1686 mention of "Fooles holy day" is the earliest explicit reference to the holiday known in England. There are various theories about its origin but they all seem specious at best.
The first film I know of that dealt with April Fools' Day was Shirley Mason's 1911 short, April Fool. Fred Walton starred in a short of the same name in the same year. 1918's Wharton film followed. In 1920, April Fool, starring Charley Chase, was released. 1924's film of the same name dealt with a newspaperman named Jimmy Jump. Hilarity ensues when the staff have difficulty distinguishing April Fools' jokes from actual events. There's also a 1926 film of the same name, about a man who operates an umbrella business.
In 1954, April Fools movies went international with Egypt's Kidbet April. 1964's April Fool, starring Saira Banu, Biswajeet and Jayant, was Bollywood's entry into the April Fools' subgenre. Hong Kong joined the fray with Chu hu yi liao (1975) and in 1989, Israel released Ehad B'April.
Back in the USA, Jack Lemmon co-starred with Catherine Deneuve in The April Fools (1969).
After a series of holiday-themed slasher films, the Christmas-themed Black Christmas (1974), the Halloween-themed Halloween (1978), the Valentine's-themed My Bloody Valentine (1981) and the Thanksgiving-themed Home Sweet Home (1981), three April Fools slasher films were released in 1986; April Fool's Day, Killer Party and Slaughter High. April Fool's Day was directed by Fred Walton, the very same Fred Walton who starred in the 1911 film, April Fool! (April Fools)
Years later, April Fools (2007) followed their inspiration, albeit with an "urban" twist.