There were two apartments in the sanctuary: The Holy Place, and the Most Holy Place.
This is exactly what Paul was previously describing at the opening of the chapter building up to the verse in question:
Then indeed, even the first covenant had ordinances of divine service and the earthly sanctuary. For a tabernacle was prepared: the first part, in which was the lampstand, the table, and the showbread, which is called the sanctuary; and behind the second veil, the part of the tabernacle which is called the Holiest of All, which had the golden censer and the ark of the covenant overlaid on all sides with gold, in which were the golden pot that had the manna, Aaron’s rod that budded, and the tablets of the covenant; and above it were the cherubim of glory overshadowing the mercy seat. Of these things we cannot now speak in detail.
Hence, Paul is saying that the first apartment ministry, the outer apartment, is symbolic of his day. The second apartment, or inner apartment, is symbolic of the fulfillment of the Day of Atonement, which is the day of judgment.
For more on this, I recommend the following resource:
God Drew the Plans - A study on the Hebrew sanctuary and its significance toward the plan of salvation. See the Related Media for video/audio studies over the sanctuary.
The sanctuary is a blueprint for the plan of salvation. Paul illustrates this throughout the book of Hebrews. Just as the Old Covenant had a sanctuary, the New Covenant has a heavenly sanctuary (Hebrews 8:1-12). We also have a High Priest, even Jesus Christ, who ministers not the blood of animals, but His own blood. It is a most wonderful study.