This unique expression occurs nowhere else in the Old Testament. It does, however, express the Hebrew understanding of a day starting on the evening (see Genesis 1:5). Understood in this way, this Day of Atonement began on the evening of the ninth day of the seventh month and concludes on the evening of the tenth day.
32 It shall be to you a Sabbath of solemn rest, and you shall afflict yourselves. On the ninth day of the month beginning at evening, from evening to evening shall you keep your Sabbath.”