How often could the high priest enter the Most Holy Place behind the veil?
The high priest could enter the Most Holy Place behind the veil only once a year, specifically on the Day of Atonement. He was not permitted to go in at all times or every day, because God's presence dwelt there in the cloud upon the mercy seat.
Les teaches that God instructed Moses to tell Aaron not to come at all times into the holy place within the veil, before the mercy seat, so that he would not die. The high priest could enter behind the veil only once a year, on the Day of Atonement. This was because the Shekinah glory was there upon the mercy seat, the very presence of God. Les also notes the priest had to undergo cleansing rituals, just as the Passover lamb had to be perfect without blemish. This pictured Christ, who fulfilled these types when He went to the cross.