Brewer's: April Fool
Called in France un poisson d'Avril (q.v.), and in Scotland a gowk (cuckoo). In Hindustan similar tricks are played at the Huli Festival (March 31st). So that it cannot refer to the uncertainty of the weather, nor yet to the mockery trial of our Redeemer, the two most popular explanations. A better solution is this: As March 25th used to be New Year's Day, April 1st was its octave, when its festivities culminated and ended.
For the same reason that the “Mockery of Jesus” is rejected as a solution of this custom, the tradition that it arose from Noah sending out the dove on the first month may be set aside. Perhaps it may be a relic of the Roman “Cerealia,” held at the beginning of April. The tale is that Proserpina was sporting in the Elysian meadows, and had just filled her lap with daffodils, when Pluto carried her off to the lower world. Her mother, Ceres, heard the echo of her screams, and went in search of “the voice;” but her search was a fool's errand, it was hunting the gowk, or looking for the “echo of a scream.”
Of course this fable is an allegory of seedtime.