Charles Ellicott Commentary Leviticus 16:31

"It is a sabbath of solemn rest unto you, and ye shall afflict your souls; it is a statute for ever." — Leviticus 16:31 (ASV)

It shall be a Sabbath of rest to you. —Literally, a resting day of solemn resting, a Sabbath of Sabbaths, i.e., a day of complete and perfect rest. This phrase, which occurs six times in the Bible, is only applied to weekly Sabbaths (Exodus 16:23; Exodus 31:15; Exodus 35:2; Leviticus 23:3), the Day of Atonement (Leviticus 16:31; Leviticus 23:32), and to the Sabbatical year, or the year of Jubilee (Leviticus 25:4), but not to the other festivals.