Leviticus 16:2

Enter not. No one but the high priest, and he but once a year, could enter into the sanctuary: to signify that no one could enter into the sanctuary of heaven till Christ our high priest opened it by his passion, Hebrews x. 8. (Challoner) --- When the tabernacle was to be removed, and when he had to consult the Lord, he might also enter, arrayed in his pontifical attire. If the high priest was prevented by any legal uncleanness, the next priest was substituted to perform his office. (Josephus, [Antiquities?] xvii. 8.) Adjutor vicarius propter cognationem ei datus est.