The Midrash, Chapter “Acharei Mot (After the Death Of)”: On the first day of the month of Nisan, after the death of Nadab and Abiyu, the Lord commanded Moses to warn the priests not to enter the Holy of Holies but only to perform one service or another.
Even Aaron who was the great priest was not allowed to enter the Holy of Holies except for Yom Kippur and only in order to perform his duty there.
The Holy of Holies is the vessel inside a person that is totally purified of the ego. Such a corrected, pure state can be attained only once a year and only when a person ascends to the level of the great priest.
Therefore, Aaron entered the Holy of Holies only once a year and no one else was allowed to enter, since they had not purified themselves as yet and had not attained the level of the great priest.
If we translate this to the language of Kabbalah, only a person who has desires that can reach the level of the great priest can enter the Holy of Holies.
Holiness is the attribute of bestowal that is made of two levels, Hassadim and Hochma. If you don’t want to use your egoistic desires, it is called Holiness. But if you want to use even your old egoistic desires in order to bestow, it is called the Holy of Holies. When you attain this level, you become the great priest.
All this is gradually revealed along the spiritual path. When I began to study the wisdom of Kabbalah with my teacher Rabash, I wrote down every word he said. I approached everything like a scientist and saw that he regarded it scornfully, “What difference does it make if I told you about something or not?”
He was right, because if you follow the spiritual path, you discover this anyway and see, hear, and know. After all, the basic property of the spiritual levels is that when each of them is revealed, you begin to see, hear, and understand what is going on inside it.