From the Rebbe’s explanation:
– Avraham’s circumcision took place on Yom Kippur (Pirkei d’R. Eliezer, Chapter 29), and therefore the 12th of Tishrei was the third day after his Bris Milah. The Torah reveals us what happened on that day: “Now haShem appeared to him in the plains of Mamre” (Bereshit, 18:1) and informed him that he would have a son Yitzchok.
The birth of Yitzchok was completely above nature, for both Avraham and Sarah were, in the normal order of things, far beyond the age of child bearing.
The lesson from this is as follows:
A Jew, when venturing into the world to spread Judaism and Chassidus, may see that in the normal way of things, it is impossible to change the world. The story of Yitzchok’s birth provides the answer. One must not succumb to the impression or influence of the natural order of things – there are events that are not defined by it. Such as the birth of Yitzhak – the birth of yet another Jew, the event that was higher than nature.”
Source: The Rebbe’s Talk on Shabbos Parshas Haazinu, 12th Day of Tishrei, 5742.