Why is the month of Cheshvan called Mar Cheshvan?

Greetings and blessings.

The matter of the Kiddush Levana (sanctification of the moon) is such that through the moon and the other celestial bodies that continuously perform their roles, we witness the greatness of Hashem. We bless the true act of Hashem, who is the true God, and our action is true, meaning that the action itself also exists in perpetuity, as stated, "...and it shall stand forever." The essence of the blessing is directed solely to Hashem, and we must not, heaven forbid, bless or pray through the moon, but to witness His mighty deeds. This is why we say, "Aleinu Leshabeach" (It is our duty to praise) after Kiddush Levana, to clarify that we do not mistakenly imply that we honor the moon and thus we conclude that Hashem is the God above in the heavens and there is no other. Furthermore, our act of going out is merely to see the might of Hashem that He illuminates through His will to all inhabitants of the world, as it states, "Lift high your eyes and see who created these." And it is written in the Talmud that Abaye says one must stand during this prayer. The reason we recite this every month is that each month we observe the renewal of God's creation through the moon.

Additionally, there is a great secret regarding this matter: the blemish of the moon and its fullness is a profound secret in Kabbalistic thought, which cannot be expanded upon within this framework. The essence is that since the sun and the moon complement the spheres of Beauty and Sovereignty in the world of Atzilut (the world of emanation), we pray for the complete restoration that will come when the sphere of Sovereignty returns to a high level like the sphere of Beauty, just as it was before the first sin, at which point the sovereignty does not have any independent substance. All this is in anticipation of complete restoration. The reason it is also called Mar Cheshvan is for two reasons: one, because there is no holiday in this month and no joy, but there were many bad and bitter troubles for Israel.

On the 15th of Cheshvan, Jeroboam son of Nebat celebrated from his heart and Hashem's anger was kindled against Israel; and on the sixth or seventh of the month, the Chaldeans slaughtered the sons of Zedekiah, king of Judah, before his eyes, and the eyes of Zedekiah, the king of Babylon, were blinded and he was bound in copper and brought to Babylon; and in this month, the Holy One Blessed Be He punished the generation of the Flood and flooded the world and all that was in it - all that ten generations of giants labored over for a thousand and twenty-six years, remained only Noah; therefore, this month is called Mar Cheshvan, the month that was bad and bitter. The second and primary reason we call the month Mar Cheshvan is because of the water, that is, the rain for which the world is thirsty and awaits in this month. 'Mar' means a drop of water, as it is said, 'The nations are like a drop in a bucket' (Isaiah 40). In the language of our sages, it is almost never found as 'Cheshvan' but rather 'Mar Cheshvan.' This is the original name of the month.

According to the simple meaning, 'Mar Cheshvan' is a translation of 'the eighth month': a lack that is destined to be filled. And although there is no holiday in the month of Cheshvan, it was indeed fitting for there to be a holiday during the days of King Solomon. 

All of Israel was awaiting the consecration of the house, and Solomon was expecting that Hashem would command him regarding its consecration - but Hashem did not command. The Temple was locked for twelve months until Tishrei came, and he was commanded to celebrate the consecration of the Temple. Therefore, it can be said: Cheshvan lost its holiday and Tishrei gained it. With blessings, Benjamin Shmueli