Why is the ninth month called September?
Jewish holiday, 1854, from Mishnaic Hebrew yom kippur (in Biblical Hebrew, yom kippurim), literally "day of atonement," from yom "day" + kippur "atonement, expiation."
In Judaism, the Day of Atonement, the most important religious holiday; a day of fasting to atone for sins. It comes in autumn. (See Rosh Hashanah.)