Хасидус к Шмот 19:18

Therefore, the children of Israel must not eat the raised tendon: It is known about the secret of ashan - which is, world/year/living being (designated by the first letter of these three words, olam/shanah/nefesh) - that there is a complete stature to each of them from the 248 limbs and the 365 tendons, in the way of their structure. It is as the Raavad wrote in his commentary on Sefer Yetzirah on the verse (Exodus 19:18), "The entire Mount Sinai was all smoke (ashan)" - that the word, ashan, hints to olam/shanah/nefesh. Since just like there is structure to a living being from the 248 limbs and the 365 tendons, so too is there in the world - as we have found that the earth is referred to by the name of limbs, the navel of the earth, the mouth of the earth, the nakedness (genitalia) of the earth. And likewise with the year, the days are distinguished; such that each one of the 365 days is the aspect of a specific tendon. So the day of the 9th of Av is the aspect of the raised tendon from the stature of the year, which is the aspect of the rear, that is where the shells (klipot) have a hold. Hence it is the time of their control, since the first and second Temples were destroyed, Beitar was captured and [Jerusalem] was plowed over because it is the day of their control and their hold. For that day is given to them, that it should be a hold even in the stature of the year. It is just like Yom Kippur, which is an exalted day and is sanctified by a sanctification of [its] level over the entire year; since it is the life of the whole year and the choicest of all of them for holiness - such that this is why the Satan is not given permission to accuse. It is as our Rabbis, may their memory be blessed, said, "This corresponding to that, did God make." For the Ninth of Av is the chosen day for the other side (sitra achra, the side of evil), to be a great hold for [its forces] on this day. For it is because of this that what happened, happened.