Wiktionary
n. 1 (context dated nautical idiomatic English) In British naval tradition, this originally referred to a day of the week when galley kitchens served no meat on board ship. 2 (context idiomatic English) In modern usage it refers to a picnic or cookout for the ship's crew.
Usage examples of "banyan day".
Today's fare, by immemorial custom, should have been half a pint of dried peas and oatmeal, this being a banyan day.
The men had a pretty clear idea of which way the wind blew by this time, and on the next banyan day, when butter alone was served out, they refused it, saying that to accept butter without cheese would be a tacit acknowledgment of the theft.
But since today is a banyan day I have ordered beef to be served out instead of the dried peas, as being a better foundation for battle.
Having come about, the squadron made a fair board, quite time enough to eat their quarter of a pound of cheese (this was a banyan day), and then returned to the former course, though with a little more west in it.