"That he may not come at all times into the holy place"Leviticus 16:2.

The high priest is forbidden from entering the Holy of Holies any time other than Yom Kippur, and only when he is then performing the special Yom Kippur service.

The regular priest is forbidden from entering the Holy Temple's sanctuary unless he is discharging a service there [e.g., kindling the menorah, offering incense, or simply entering to prostrate himself before G‑d].