This past Shabbos was parshas Shmini, which involves the key discourse by the Lubavitcher Rebbe, where he asks what was missing in the 7th day of completion, and what was added by the 8th day that wasn't there before.
The Rebbe, in this obscure discourse from 1977 (5737), connects this with the 7 generations of Chabad, and the famous discourse beginning the Rebbe's generation in 1951 (5711) quoting the Medrash:
"in the beginning the Essence of the Divine Presence dwelled in this physical world, and with the sin of Adam it departed to the first firmament, and with the following sins it departed all the way to the 7th firmament. Then 7 tzadikim stood and brought the Divine Presence back, Avraham from the 7th firmament to the 6th, Yitzchak from thr 6th to the 5th, until Moshe who is the 7th, and all the 7ths are beloved, brought the Divine Presence down to Earth"
However, even after untiing Heaven and Earth by Moshe Rabeinu in Matan Torah, the Divine Presence was not openly revealed to all until the 8th Day - following the 7 Days of Competion. And the Rebbe asks what was missing and what could it possible be that Moshe Rabeinu can not accomplish and he needs to give it over to Aharon, his sons, and the Nation of Israel?
The answer is that in order for the Divine Presence to openly manifest within the physical and material world, within the constraints of Time and Space without nullifying Time and Space, the revelation must come from WITHIN the physical world, and not forced upon it from Above.
Moshe Rabeinu was Ish HaElokim - the ultimate human state that is openly manifesting its G-dly root. Since G-dliness was openly revealed by Moshe Rabeinu, the physical world and its contraints of Time and Space do not truly exist - they are nullified in the G-dliness which is vivifying reality at every instant anew. This is also true for the other True Tzadikim (which are few) in every generation, where their awareness and influence transcends Time and Space.
However, the "fault" in transcending time and space is that it makes it impossible to reveal G-dliness WITHIN time and space. The miraculous state of Moshe Rabeinu actually makes it impossible for him to act within with the physical, since physicality (and the yetzer hara that comes with it) "runs away" from Moshe Rabeinu.
That's why, while Moshe Rabeinu was able to unite Heaven to Earth, for open revelation of G-dliness within the world requires "regular" people who "live within the world" - that's us. This is symbolized by Aharon - whose divine service is of the sacrifices - elevating the physical material world (through its 4 aspects of: inanimate, vegetative, animal, and man) in a manner of "from below to above".
However, how can Aharon elevate the word? Through the transfer of power from Moshe to Aharon and to his sons and the Elders of Israel - which is the Calling of Moshe to Aharon on the 8th Day.
I have written before about how this relates to the giving over of power by the Rebbe to the Chassidim and the Nation of Israel on the 28th Nissan 1991 (5751). You can check it out in detail in those posts. However, where this concept is left off by me is - what exactly do we need to do this now to make this happen, and openly reveal G-dliness in the world? This is the divine mission of the 8th generation, which is the 10th generation to the Baal Shem Tov (on that 8th day 10 crowns were given as Rashi says).
So, how do we do this? Well, first - what has changed?