Read Exodus 30:10 Using Other Translations
And Aaron shall make an atonement upon the horns of it once in a year with the blood of the sin offering of atonements: once in the year shall he make atonement upon it throughout your generations: it is most holy unto the LORD.
Aaron shall make atonement on its horns once a year. With the blood of the sin offering of atonement he shall make atonement for it once in the year throughout your generations. It is most holy to the LORD."
“Once a year Aaron must purify the altar by smearing its horns with blood from the offering made to purify the people from their sin. This will be a regular, annual event from generation to generation, for this is the LORD ’s most holy altar.”