When and how did April Fool's come into being? [SERIOUS]

According to the secondary literature, we don't know for certain but we have some good ideas [see, for example, McEntire, Nancy Cassell. "Purposeful Deceptions of the April Fool." Western Folklore 61, no. 2 (2002): 133-51.]

One of the deleted comments suggested that it was due to the Gregorian calendar reforms, which the French adopted soon after the papal bull was promulgated in 1582. Under the new calendar, it is supposed, the French began recognising the new year on 1 January: the traditionalists who insisted on marking the start of the year on 25 March were, therefore, the fools.

This is likely incorrect. 1 January marked the start of the new year under the Roman tradition, at least since the Julian calendar reform, and was inconsistently used throughout Europe through to the early modern era. And it was set as the start of the year in France with the Edict of Roussillon in 1564 -- as such, the French were unified in observing the new year on the 1 January several decades before the Gregorian calendar reform.

More generally, it seems that people tend to get a bit festive and prankish as winter transitions into spring. McEntire, cited above, refers to Holi (India), but there's a reasonably well-referenced article on Hoaxes which refers to a few other examples. We also have the example of the Flemish poem "Refereyn vp verzendekens dach Twelck den eersten April te zyne plach (Refrain on errand-day which is the first of April)" which is purportedly about sending a servant on a fool's errand -- the late-medieval equivalent of sending someone off for a left-handed screwdriver. Composed in 1561, the poem hints at what was already an established tradition.

Most likely, this is the origin for the holiday. A socially-sanctioned day in which you can prank people, which is celebrated in the month which marks the seasonal transition out of winter (compare with Halloween, which is about the seasonal transition into winter). Calendar reform is a bit of a red herring, in this sense. I'm a bit surprised a cultural historian hasn't done a deeper dive on this, though: perhaps a few have tried and came up empty.