The tenth of the seventh month — the one day a year anyone entered the holiest place, and the whole nation fasted.
The most solemn day in the calendar: a fast, no work of any kind, and the only day the high priest went behind the veil, carrying blood for himself and for the people. Two goats were taken — one killed, and one sent away into the wilderness carrying the nation's guilt, which is where the word scapegoat comes from. Hebrews spends several chapters on this one day, because the argument it makes about a better sacrifice is built on it.