According to the OED, the term is borrowed from the Banyans in the East Indies, a caste that ate nothing that had life.
banyan day (plural banyan days)
- (dated, Britain, nautical, idiomatic) In British naval tradition, a day of the week when galley kitchens served no meat on board ship.
- 1819, James Hardy Vaux, Memoirs of James Hardy Vaux, Chapter XVIII, p. 204–205:
- This was a favourable circumstance in one respect to myself and the ship's company, for as Tuesday is a sumptuous day in point of allowance in the navy, beef and pudding being the prescribed fare for dinner, we by this accident feasted two days together; whereas had it occurred on a Monday, Wednesday, or Friday, two successive banyan (or starvation) days would have been our dismal portion.
- (Britain, nautical, idiomatic) A picnic or cookout for the ship's crew.