The Rebbe's guidance on Sukkos, the Four Species, Shemini Atzeres and Simchas Torah:
The sukkah. Dwelling in the sukkah is not only physical; it calls for awareness that one is surrounded by holiness, and it shows how Torah and mitzvos must permeate every part of life after Yom Kippur (IK 780, 171). There is no basis for preferring a sukkah placed outside the house (IK 3874). Longstanding customs, such as about sleeping in the sukkah, should be investigated respectfully rather than dismissed (IK 3796). Bringing students into the sukkah leaves a lasting impression (IK 3839). On the order of Shehecheyanu and Leshev BaSukkah on the second night, see IK 1227.
The Four Species. The Four Species express the unity of all Jews (IK 105, 781, 533), and sharing one's set so others can fulfill the mitzvah brings great merit (IK 5816, 7049). Chabad's custom is to use Calabrian esrogim (IK 590, 6293); a reliable tradition establishes that an esrog is not grafted even without a pitom (IK 2998, 8838), and one should seek esrogim from traditional sources (IK 3149, 3547). Shake the lulav only at the first "Hodu LaShem" (IK 2483); on the direction of the na'anuim, see IK 3816.
Joy. Sukkos embodies a double and triple joy — our joy in G-d and His Torah, and His joy in us (IK 532, 9229). The joy of Simchas Torah should be drawn into the entire year (IK 782, 783, 8892), and true joy breaks through every barrier (IK 1236). Simchas Torah unites all Jews, especially when celebrated in Jerusalem (IK 9579, 9760, 10237, 10568). After Simchas Torah, each of us sets out on his own path, like Yaakov (IK 7457).
Hoshana Rabbah. A time for communal prayer, including for members of Tehillim groups (IK 7770, 7792).
See also: Teshuvah – Elul, Rosh Hashanah and the Ten Days of Teshuvah; Jewish outreach – Mitzvah campaigns.