The six monthly posadha, or fast days: the 8th, 14th, 15th, 23rd, 29th, and 30th. They are the days on which the Four Mahārājas 四天王 take note of human conduct and when evil demons are busy, so that great care is required and consequently nothing should be eaten after noon, hence the 'fast', v. 梵王經 30th command. The 智度論 13 describes them as 惡日 evil or dangerous days, and says they arose from an ancient custom of cutting of the flesh and casting it into the fire.

A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous