We should understand why we must sanctify specifically the moon.
It is known that sun and moon imply Zeir Anpin and Malchut. With respect to branch and root, we pray during the sanctification of the moon, “May it please Him to fill the blemish in the moon.” We should understand why we should care if there is a blemish in the moon, that we are told to sanctify the moon and ask the Creator to fill her blemish. Also, what is her blemish?
From the words of our sages, “Bring upon me atonement for I have diminished the moon” (Hulin 60b), it is implied that the Creator sinned when He diminished the moon. We should understand what is this sin that we must correct.
The thing is that it is known that the moon implies Malchut, meaning the kingdom of heaven. In other words, although we should believe that “The whole earth is full of His glory,” there was a Tzimtzum [restriction] and concealment so we do not feel it, and we must believe above reason that this is so.
Thus, it is as though the concealment that the Creator has made caused the creatures not to be able to take upon themselves the kingdom of heaven due to the concealment. But who made the concealment? It is the Creator. Hence, this is similar to the prohibition, “Do not place an obstacle before a blind man.”
Indeed, why did He make the concealment? It is as Baal HaSulam interpreted, that without the concealment, it would be utterly impossible to make a choice to receive in order to bestow. Instead, all the creatures would be serving the Creator in a manner of receiving in order to receive. But since He made the concealment and Tzimtzum, there is room for choice, for the creatures to work in order to bestow.
It follows that if the lower ones bring an offering, meaning they offer themselves to the Creator, meaning equivalence of form, everyone sees that thanks to the concealment it was possible to achieve equivalence of form.
Thus, concealing and restricting Malchut was not a sin. On the contrary, only the concealment caused all this. It follows that the flaw passed away. This is the meaning of what he said, “Bring upon me atonement,” for atonement comes from the word “bad,” “to wipe My hand on that man,” meaning that all the flaw was erased.
Therefore, it follows that when we sanctify the moon it is that through the flaw and concealment it became possible to begin to work in order to bestow. Thus, the concealment enters the Kedusha [holiness]. It follows that this is not regarded as a flaw and a lack but is regarded as a correction. This is called “the sanctification of the moon,” and by this the blemish is filled, meaning that we see that it was not a blemish whatsoever.