In Moses' time, the sanctuary was cleansed once a year on the Day of Atonement.
(Leviticus 16:30 KJV) For on that day shall the priest make an atonement
for you, to cleanse you, that ye may be clean from all your sins
before the Lord.
On this day, two goats are chosen. One is sacrificed on behalf of the congregation for their sins, and the blood is sprinkled inside in the holy place inside the veil, on the mercy seat (Leviticus 16:15). Sin is transferred from the sinner, through the blood of the sacrifice, into the Sanctuary.
Once the High Priest has made an end to reconciliation, he walks in the courtyard to the other goat (the live goat) (Leviticus 16:20):
(Leviticus 16:21) And Aaron shall lay both his hands upon the head of the live goat,
and confess over him all the iniquities of the children of Israel, and
all their transgressions in all their sins, putting them upon the head
of the goat, and shall send him away by the hand of a fit man into the
wilderness: 22 And the goat shall bear upon him all their iniquities unto a land not inhabited: and he shall let go the goat in the wilderness.
By this act the sanctuary is fully cleansed, with sin removed from the sanctuary by the merits of the spotless sacrifice, and resting ultimately on the Scapegoat (or Azazel in Hebrew).
Now, through Hebrew 8, we know that the earthly sanctuary is merely a copy of the true Sanctuary in Heaven. All patterns and ceremonies are merely an enactment of the true services in Heaven.
In the Heavenly sanctuary, Jesus is both the spotless sacrifice and our High Priest. In fact, the true mercy seat exists in the Heavenly Sanctuary, on the ark of covenant in heaven; "And the temple of God was opened in heaven, and there was seen in his temple the ark of his testament." (Revelation 11:19). Thus in the heavenly sanctuary, services are performed to address sin that were represented by the services on earth.
When we sin and are forgiven, our sins are removed through the blood of the sinless sacrifice. However, they do not disappear, but are transferred by the blood into the Heavenly Sanctuary. The book of remembrance keeps a record of all our sins until the Judgement. The Day of Atonement represents a period in heaven when the Sanctuary is cleansed. Sins are blotted out from the book of remembrance through the merits of Jesus, and will be placed ultimately unto the Scapegoat. By this, justice is served.
(Daniel 8:14) And he said unto me, Unto two thousand and three hundred
days; then shall the sanctuary be cleansed.