The earliest recorded association between 1 April and foolishness can be found in Geoffrey Chaucer's The Canterbury Tales (1392).
According to this article on The Museum of Hoaxes, if Chaucer really did allude to April Fools' Day in the 1390s, it would have been more than a hundred years before any other reference to the unofficial holiday.
... most Chaucer scholars do not believe the tale is set on April 1 or 2. Instead, the majority of them (almost all) believe the tale is set on May 3.
The article offers a more likely first reference to the day in the form of an Eduard de Dene poem, which ends with a servant realizing the errands he's been forced to do on the 1st of April are senseless.
This appears to be the first clear reference to a custom of playing practical jokes on April 1st. Because of this reference, historians believe that April Fool’s Day may have originated in continental northern Europe and then spread to the British Isles.
Check out the full article here.
It's a good read if, like me, you're going out of your way to avoid the Rickrolls and Fallout 4 "pranks" which are likely running rampant today on community-driven sites.