The Rebbe reveals:
– One of the unique sources of happiness on Sukkot is the presence of the ushpizen – the supernal guests, “founding fathers” of the Jewish people, who come to visit us in the sukkah (both those mentioned in the Zohar and the Chassidic ushpizen mentioned by the Rebbe Rayatz).
Yaakov Avinu – the primary ushpiz of the present night shares a particular connection with the holiday of Sukkot for the Torah specifically mentions him in association with Sukkot:
And Yaakov traveled to Sukkot and built himself a house, and for his cattle he made booths; therefore he named the place Sukkot … (Bereshit, 33:17).
Rashi: He stayed there eighteen months: summer, winter, and summer. “Sukkot” denotes summer. “A house” denotes winter, and [again] “Sukkot” denotes summer. — [from Gen. Rabbah 78:16]
Yaakov Avinu is associated with the service of Torah study, and so is we find in the Book of Tehillim:
And He established testimony in Jacob, and He set down a Torah in Israel, which He commanded our forefathers to make them known to their sons. (Tehillim, 78:5).
His influence is complemented by the Alter Rebbe – Rabbi Shneur Zalman of Liadi, the chassidic guest who comes to visit us in the sukkah on the third day of the festival. His first name, Shneur, conveys the unity of the two lights of Torah: Nigleh, the teachings of Torah law, and Pnimiyus HaTorah, the Torah’s mystical dimension.
His second name, Zalman, shares the same Hebrew letters as the word l’zman (“into time”), meaning that the two lights of the Torah are drawn down “into time”, that is internalized within the context of our material world.
From the Rebbe’s talk on the 3d night of Succot, 5751.