The way I understood it april fools came from the Netherlands.
At the first of april in the year "long ago" (forgot my history
) the Dutch city of Den Briel was captured by the Watergeuzen. They did so not by brute force but by fooling the Spanish forces. Overnight the Dutch came in on a supply vessel. They raised the Dutch flag on the church, and rang the church bells. The Spanish troops saw the flag and though they had been overrun, and fled.
Anyway, it's been long since I had history class so I might be slightly off.