1. Lechatichila Ariber - from the onset, go above.
The sicha was said on the day of the Hilula of the Rebbe Mohorosh, the 4th Lubavitcher Rebbe, who was famous for saying "the world says that when you can;t go under, go over, I say that from the onset you should go over."
This seemingly "cute" idea holds a much deeper concept. Mainly, that while the world is limited by time and space. both are created being, and the G-dliness that recreates the world from Absolute Nothing is by definition above the limitation of time and sapce.
Most, if not all, of us live within this limitation of time and space. Only under extenuating circumstances do we feel so stressed that we tap into the reality above time and space. The Rebbe Mohorosh tell us to from the beginning work with our intrinsic G-dly qualities, totally above time and space. See more on this in the sicha 13 Tishrey 5744.
Until the time of the redemption, people are bound by the limitation of time and space, while very few in each generation who are complete tzadikim were at times acting beyond time and sapce (Lechatchila Ariber). The Rebbe in this sicha is telling us that each and every one of us should act like complete tzadikim, hence act lechatchila ariber.
But how could this be? if even in Tanya it says that there are indeed very few tzadikim in each generation, and most of us are quite far from being tzadikim, mush less complete tzadikim. But the Rebbe explains elsewhere in the discourses of 1991-1992 that until the time of the end of the sifting of the 288 sparks, we had no choice and were bound by nature. But after the time of the complete sifting of the sparks in 1991 (5751) each and every one of us has the ability and obligation to elevate to the level where we are not associated with our personality first and foremost, but rather associated with our Neshama. This is the level of tzadikim.
But how come we don't see a difference in the world and inside of us since 1991? That's because of our habits - our habitual association with our personality. Our viewing the world through the eyes of our personality and not through the eyes of our Neshama.
The work at hand is therefore a work of breaking habits and getting used to new habits. This is a function of learning and implementing our learning into a new way of looking at our life.
What comes out of this is that Lechatchila Ariber is not just a nice saying, it is a path in divine service and the path for bringing about the redemption by Moshiach. It embodies all of the teachings and effort of the Torah of Chassidus since its onset and is the Rebbe's direction to our generation.
This, in my opinion, is the introduction to the Rebbe's message regarding Hakhel, because (a.) this is the way to implement such a lofty goal as Hakhel - to gather the Jews globally into a central network, (b.) it is the individual mission of every Jew int heir process of personal redemption, for which Hakhel is the ideal support.
The Rebbe actually says in the sicha:
(end of ch. 6):
"... we stand in the year of Hakhel which causes an elevation (ariber) in all of Israel, the men women and children, until the very infants, are all elevated by the ingathering of the exiles together, which has an affect on the very place where they gather (even at the time that the physical Temple is not yet standing), that the place should be a holy place [holy means above time and space, unified with G-d]..."
(beginning of ch. 7):
" ... Lechatchila Ariber quickens and hastens the true and complete redemption by Moshiach, since when we are talking about a person that is above "cheshbonos" [above limitation of logic and reason of time and space] then there is no room of reasoning against him, and the redemption is something he deserves immediately..." Very deep words, I think the Rebbe is saying that when a person is in the state where he is bound by time and space, then these limitations limit his ability to "merit" which means to relate to the redemption. When the person elevates above time and space and hence limitations, he automatically "merits' which means associated with the dimension of the redemption which already exists but has to be "brought down" to the real world in a practical way.