Brewer's: Poisson d'Avril
An April fool. The poisson d'Avril is the mackerel, and we have the expression “You silly mackerel,” and silly indeed are those who allow themselves to be caught by the palpable jokes engendered on the 1st of April. The Scotch say “hunting the gowk” (cuckoo). It is said that the best explanation is a reference to Matt. xxix. 2.
The mackerel, says Oudin, is called the poisson d'Avril, “parce que les macquereaux se prennent et se mangent environ ce mois-la.”
A correspondent of Notes and Queries (June 20, 1891, p. 494) says that the April fish is the aurata, sacred to Venus.