When Jesus returned to Heaven he was coronatied as our High Priest and King. With this dual function He sits on the Father's throne as King while also serving in the heavenly sanctuary as our High Priest. The last part of the ministry of the priest was to perform the Day of Atonement. The writer of the book of Hebrews who wrote the book about AD 60 says

Heb 10:25 "not abandoning our own meetings, as some are in the habit of doing, but encouraging each other, and even more so because you see the day drawing near."

The Day spoken of here is the day of Atonement. Read this article written by Theologian Dr. Richard Davidson on the ministry of Christ in the heavenly sanctuary. Showing that when Jesus returned to heaven he inaugurated the entire heavenly sanctury for service  also showing how the day of atonement did not begin at the cross but was still written of as being a future event when the book was written in AD 60. Hebrews chapter 9 is not about the Day of Atonement but is all about the antitypical Day of the inauguration of the sanctuary. The transition between the Old Covenant sanctuary service and the New Covenant sanctuary service.