I guess this crosses over with the “Middle French Translation Thread” but will post here.
French is not the focus of this forum, but thought you might want the info in case you update the Big List entry.
The History of Poisson d’Avril is not quite right here. The earliest recorded use is from 1466 “Doctrinal du temps présent” by Pierre Michault, described as a “go-between, intermediary, young boy responsible for delivering his master’s love letters” (my translation, so beware!).
Not at all clear if this is already a deliberate conflation/play of the words for pimp and mackerel, no solid explanation for the term as a messenger.
But even so, how it arrived at it’s modern “april fool” use is equally murky. Maybe as the jokes were originally to send the victim on a stupid errand?
Also the translation of “vien tost a moy”, I would suggest “come soon to me”
As for April Fool itself, another conjecture is that is arose out of other odd religious traditions around that time of year, even going back to the Roman “Hilaria” equinox celebrations which also had a “comic” element.