Midrash Raba, Shlach: The evening of the unfortunate complaints will be the ninth of Av. And the Creator ruled: you have cried in vain, for no reason, and therefore I will give you a reason for your weeping. The ninth of Av went on to become a day of mourning for the whole nation. It is actually on this day that the first and second temples were destroyed…
This is actually what has happened until this very day. If we review all of our history, we will see that the hardest events occurred on the ninth of Av: the expulsion from the countries the Jews lived in, like Spain, for example, and the plans of Hitler’s supporters to destroy the Jews, and so forth.
Every spiritual root has a parallel branch in our world. All of this takes place in the spiritual world in the opposite manner from the corporeal world, and so in our world the ninth of Av is etched as a day of mourning.
We don’t even study the Torah on the ninth of Av because the Surrounding Light cannot be conveyed unto us during this time.
The root of the ninth of Av will exist until the end of correction, and only then will it become the greatest day since it is actually the revelation of the greatest ego that will take place that day, and we will be given the greatest urge, the revelation of the world of Ein Sof (Infinity) in our world.
The deepest descents will turn into the greatest ascents. The two worlds, the uppermost and the lowest, will connect.
From KabTV’s “Secrets of the Eternal Book” 4/15/15