Many Adventist scholars agree that Hebrews is speaking about the Day of Atonement. Here are a few brief quotes on the subject.

Seventh-day Adventist Bible Commentary on Hebrews 10:1:

Compare ch. 9:25, 26, where the work of Christ is again contrasted with that of the earthly high priest on the Day of Atonement.

M.L. Andreasen in The Book of Hebrews:

On Hebrews 4:16

Verse 16. "The throne of grace." This expression in Christian terminology has always been closely
connected with prayer, and hence with the mercy seat. It was at the mercy seat the high priest supplicated
God for forgiveness on the Day of Atonement. We are invited to come there to find grace to help in time of
need. (64)

On Hebrews 9:25-26

Verses 25, 26. The priests entered the first apartment daily, the high priest once every year when
he went into the most holy with the blood of the bullock and the goat. (127)

William Johnsson in his essay "Day of Atonement Allusions," which can be found in the DARCOM volume on Hebrews, lists 9:25 as clearly alluding to the Day of Atonement.

The context clearly points to a Day of Atonement allusion (high priest . . . yearly . . . blood [cf. 9:7]) (113)

Alwyn Salom in his appendix article in the Daniel and Revelation committee series, speaking of verse 24, 25:

The reference in the context of the Day of Atonement service of the earthly high priest is not to the outer compartment of the sanctuary. (227)

Richard Davidson, notes that vs. 25 is an unmistakable reference to the Day of Atonement:

I agree with Young that Hebrews 9:7 and 9:25 refer to Day of Atonement, because of the clear references to “once a year” and “every year” respectively. ("Inauguration or Day of Atonement?" Andrews University Seminary Studies, Spring 2002, 79)

Felix Cortez states in his article "From the Holy to the Most Holy Place: The Period of Hebrews 9:6-10 and the Day of Atonement as a Metaphor of Transition" in the Journal of Biblical Literature, 125.3, Fall 2006, 527 (footnote):

Unchallenged references to the Day of Atonement in the central section include 9:7, 25

Despite recognizing these allusions to the Day of Atonement, the above authors largely find ways to avoid the conclusion that Jesus completed the sacrifice, entry and purification that Hebrews speaks of in the past tense. Their means for doing so do not always agree. But they in general agree that Jesus did not complete that part of the service.

Yet the very texts that they admit are Day of Atonement references are set in the past, and seen as fulfilled. The whole argument of the author is that Jesus has surpassed the old system, and the Day of Atonement is an important part of his argument.