"...they liked what they were used to, a regular life with no changes of any kind, no mad interference with the steady succession of salt pork on Sunday and Thursday, salt beef on Tuesday and Saturday, with banian-days in between; the sea itself could be relied upon to provide all the variety that could possibly be desired."
In the English Navy, a day on which salt meat was replaced by fish or cheese, a practice begun during the reign of Elizabeth I to reduce costs. The term derives from the Banians, Hindu traders who abstained from meat because they held animals to be sacred. Also, the banyan is an East Indian tree of the mulberry family... etc. (p. 99)
I know banyan can also mean a long gown worn by gentlemen in the eighteenth century as something like a bathrobe or dressing-gown. Haven't seen that meaning with the spelling banian though.