The Old Testament high priest had a yearly appointment with God in the most sacred space. But his access was limited and required a specific, recurring ritual.
The writer of Hebrews is drawing a stark contrast between the Levitical high priest and Jesus.
The Annual Ritual
The high priest could only enter the Most Holy Place once a year, on the Day of Atonement. This wasn't a casual visit; it was a highly regulated, solemn event. The purpose was to make atonement for the sins of the people.
Blood of Others
Crucially, the high priest entered not with his own blood, but with the blood of animals (goats and calves). This blood was a symbol, a temporary covering for sin, and it highlighted the inadequacy of the Old Covenant system. It pointed forward to something greater.