Rashi on Exodus 30:10/רש"י על שמות ל׳:י׳

1וכפר אהרן AND AARON SHALL MAKE EXPIATION — This refers to the placing of the blood upon the horns of the altar.

2אחת בשנה ONCE IN THE YEAR — On the Day of Atonement. This is what is said in the section אחרי מות, which describes the sacrificial service on that day, (Leviticus 16:18) “And he shall go out unto the altar that is before the Lord and make an expiation upon it”.

3חטאת הכפרים SIN-OFFERING OF EXPIATION — These are the bullock and goat of the Day of Atonement which atone for the uncleanness of the Sanctuary and its holy offerings (Shevuot 2b; cf. Rashi on Leviticus 16:11).

4קדש קדשים [IT IS] MOST HOLY — the altar is sanctified for offering on it these things alone and not for any other sacrificial service.