Hebrews 9:7

and into the second comes the high priest alone once a year, not without blood, which he offers for himself, and the errors of the people;

Hebrews 9:25

neither in order that he may frequently offer up himself, as the high priest goes into the holies once a year with the blood of another;

Hebrews 10:3

But in the same sacrifices there is a remembrance of sins annually;

Hebrews 10:14

For by one offering he has forever perfected the sanctified.

Treasury of Scripture Knowledge

Summary

And this shall be an everlasting statute unto you, to make an atonement for the children of Israel for all their sins once a year. And he did as the LORD commanded Moses.

Bible References

Once a year

Hebrews 9:7
and into the second comes the high priest alone once a year, not without blood, which he offers for himself, and the errors of the people;
Hebrews 10:3
But in the same sacrifices there is a remembrance of sins annually;