Entering the Holy of Holies in the Old Testament occurred once per year on the day of atonement. See also Lev 16:29-34.
Also on the tenth day of this seventh month there shall be a day of atonement: it shall be an holy convocation unto you; and ye shall afflict your souls, and offer an offering made by fire unto the LORD. Lev 23:27
That would be the general pattern of the reality in Christ Jesus. I say "general" because while the Old required annual sacrifices, Christ did this once for all time never to be repeated. He does not suffer often.
For then must he often have suffered since the foundation of the world: but now once in the end of the world hath he appeared to put away sin by the sacrifice of himself. Heb 9:26
But when did entering the heavenly Holy of Holies take place? Was it during His time in the tomb, ascension 40 days later, or some 6 months later at Atonement day or centuries later? Or does the bible tell us?
Recall the event between Mary Magdalene and Christ in the garden after He rose out from the dead.
Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God. John 20:17
Touch me not because He had yet to ascend to His Father. This is an oblique reference to the High Priest at Atonement.
And there shall be no man in the tabernacle of the congregation when he [Levitical High Priest] goeth in to make an atonement in the holy place, until he come out, and have made an atonement for himself, and for his household, and for all the congregation of Israel. Lev 16:17
In other words, Mary should not have been present, let alone touching Him. However, once He completed atonement, it was perfectly fine to touch.
Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing. John 20:27
There is one other piece, the requirement that the one making a Testament must die before it is in effect.
For where a testament is, there must also of necessity be the death of the testator. Heb 9:16
Christ made the Testament at the Last Supper and death. And then resurrected for our life.
So Christ was once offered to bear the sins of many; and unto them that look for him shall he appear the second time without sin unto salvation. Heb 9:28
So, to answer the OP from this Protestant point of view, when did Christ enter the heavenly Holy of Holies?
Within minutes of His resurrection, He ascended into the heavenly Holy of Holies.
TO ADD: Christ is explicit about the timing of ascending to God in John 20:17. He tells Mary, I ascend.
Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God.
"Ascend" is in the present tense. She leaves, He ascends.
Yes, that same day He also descends back to earth.
Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you. John 20:19
Later the same day, He shows them His wounds to prove Himself who He is.