what seems certain is that it is in some way or other a relic of those once universal festivities held at the vernal equinox, which, beginning on old new year's day, the 25th of march, ended on the 1st of april.
it has been suggested that europe derived its april-fooling from the french [2]. france was one of the first nations to make january 1 officially new year's day (which was already celebrated by many), by decree of charles ix. this was in 1564, even before the adoption of the gregorian calendar (see julian start of the year). thus the new year's gifts and visits of felicitation which had been the feature of the 1st of april became associated with the first day of january, and those who disliked or did not hear about the change were fair game for those wits who amused themselves by sending mock presents and paying calls of pretended ceremony on the 1st of april. french and dutch references from 1508 and 1539 respectively describe april fools' day jokes and the custom of making them on the first of april.
though the 1st of april appears to have been anciently observed in great britain as a general festival, it was apparently not until the beginning of the 18th century that the making of april-fools was a common custom. in scotland the custom was known as "hunting the gowk," i.e. the cuckoo, and april-fools were "april-gowks," the cuckoo being there, as it is in most lands, a term of contempt. in france the person fooled is known as poisson d'avril. this has been explained from the association of ideas arising from the fact that in april the sun quits the zodiacal sign of the fish. a far more natural explanation would seem to be that the april fish would be a young fish and therefore easily caught.
the french traditionally celebrated this holiday by placing a dead fish on the back of friends. today the fish is substituted by a floppy kerk.