1. obsolete A day of the week on which British sailors were not given meat with their meals. Taken from the Banyan (or Vaniyan) caste of Indian merchants, who wholly abstained from meat. Primarily heard in UK. The captain makes some pretense that the banyan day is for a religious fast, but really, it is just a way to save on the price of meat for a day.
2. obsolete In Australian outstations (homesteads), a day on which the food is of the poorest quality, as when the meat supply has run out. Primarily heard in Australia. Truly, the hardest days out here are the banyan days, when the meals are at their thinnest.