A 'spotmandement' was a kind of parody of an ecclesiastical edict or charter. These farcical documents were often published during the late-medieval period to accompany the raucous celebrations that occurred around Shrove Tuesday (the conclusion of the Carnival period before Lent began). They would use pseudo-legal language to command the celebrants to engage in behavior that would normally be forbidden or frowned upon (such as overeating, drinking, partying, gambling, loafing around, etc.).
One such handwritten spotmandement from around the mid-1500s (the exact date of its creation is unknown) detailed the plan of events for a Shrove Tuesday celebration in Jutphaas, near Utrecht in the Netherlands. It included a list of the food that was going to be eaten and the music that would be played. And then its anonymous author noted that April 1 would be the day on which fools would be required to have their fool's caps inspected:
"Dat wy die zelfden citeren omme te compareren op den eersten Aprilis naestcomende, onder de Muelen ende greffie tshoofts van Jutphaes, omme aldaer zyne Covel te demonstreeren" ("that we order the same [the fools] to appear on April 1 next, under the mill and registry of Jutphaas, to show his cap").
This single line doesn't specifically state that April 1 was a day set aside for playing pranks, but it clearly demonstrates that the author of the text associated the day with folly. And it is the earliest unambiguous reference to such an association that is known.
The Jutphaas Spotmandement is currently part of the collections of the Museum Meermanno
in the Hague.