who alone could enter the holy of holies and on what occasion
The Holy of Holies was covered by a veil, and no one was allowed to enter except the High Priest, and even he could only enter once a year on Yom Kippur (the day of atonement), to offer the blood of sacrifice and incense before the mercy seat. The Bible reports that in the wilderness, on the day that the tabernacle was first raised up, the cloud of the Lord covered the tabernacle (Exodus 40: 33-40: 34). There are other times that this was recorded, and instructions were given that the Lord would appear in the cloud upon the mercy seat (kapporet), and at that time the priests should not enter into the tabernacle (Leviticus 16: 2). According to the Hebrew Bible, the Holy of Holies contained the Ark of the Covenant with representation of Cherubim. Upon completion of the dedication of the Tabernacle, the Voice of God spoke to Moses "from between the Cherubim" (Numbers 7: 89).