Charles Ellicott Commentary Exodus 30:10

"And Aaron shall make atonement upon the horns of it once in the year; with the blood of the sin-offering of atonement once in the year shall he make atonement for it throughout your generations: it is most holy unto Jehovah." — Exodus 30:10 (ASV)

Aaron shall make an atonement upon the horns of it once in a year. —This passage seems to determine the sense of Leviticus 16:18, where some have supposed that the altar that is before the Lord is the brazen altar.

Once in the year, on the great day of atonement, the high priest, after entering within the veil and sprinkling the blood of the offerings upon the mercy seat (Leviticus 16:14–15), was to go out unto the altar that was before the Lord, and put of the blood of the bullock, and of the blood of the goat, upon the horns of the altar round about, and sprinkle of the blood upon it with his finger seven times, and so cleanse it, and hallow it, and make an atonement for it (Leviticus 16:18–19).