the Sacred Vail: The High Priest: Allowed to Enter but Once a Year
Source: TTT
Leviticus 16:2
And the LORD said to Moses: “Tell your brother Aaron not to enter freely into the Most Holy Place behind the veil in front of the mercy seat on the ark, or else he will die, because I appear in the cloud above the mercy seat.
Hebrews 9:7
But only the high priest entered the second room, and then only once a year, and never without blood, which he offered for himself and for the sins the people had committed in ignorance.