According to the (increasingly excellent) Wikipedia:
New Year was originally celebrated from March 25 to April 1, before the Gregorian reforms moved it back to January 1. The English first celebrated [April Fool's Day] on a widespread basis only as late as the 18th century, though it appears to have reached England probably from Germany in the mid-17th century. Its first known description in English originates with John Aubrey, who noted in 1686: "Fooles holy day. We observe it on ye first of April. And so it is kept in Germany everywhere."
The custom of playing practical jokes on April Fool's Day is also very widespread and of uncertain origins. The victim of a joke is known in English as an April Fool; in Scots as a gowk (cuckoo or fool); and in French as a poisson d'avril (April fish). It has been suggested the custom may have had something to do with the move of the New Year's date, when people who forgot or didn't accept the new date system were given invitations to nonexistent parties, funny gifts, etc. Originally, April Fool's Day jokes concentrated on individuals (sending someone on an absurd errand such as seeking pigeon's milk) but in the 20th century it became common for the media to perpetrate hoaxes on the general population. [emphasis added]
The Wikipedia also gives a long list of well-known April Fool's Hoaxes.
The French Wikipedia page on Poisson d'avril, taking a different perspective, asserts plainly that the practice "trouve son origine en France, en 1564... Cette coutume de faire des plaisanteries s'est répandue dans de nombreux pays, bien que le poisson ne soit pas toujours exporté en même temps."
Although there are many excellent linguistic jokes, and also quite a few interesting linguistic hoaxes, some of them pretty spectacular, I don't know of any memorable linguistic April Fool's hoaxes.
Posted by Mark Liberman at April 1, 2004 05:11 PM