There is a minhag to recite the chapter of "L'Dovid Hashem ori" twice a day from the beginning of the month of Elul until Shemini Atzeres. The Kitzur Shulchan Aruch (128:2) explains that the source for this custom is a comment of the Midrash Shochar Tov that "Hashem ori" (Hashem is my light) refers to Rosh Hashana, "yishi" (my salvation) refers to Yom Kippur and "ki yitzpineini b'sukko" (for He protects me in His sukkah) alludes to Sukkos. We know that Rosh Hashana and Yom Kippur are a unit known as the yamim noraim. But why is Sukkos included in this special grouping?
One of the central themes of the yamim noraim is accepting the kingship of the Ribbono Shel Olam. We daven that the entire world should recognize Hashem's sovereignty, and all of humanity should join together in one group - b'agudah achas - to fulfill Hashem's will. The posuk says, "And He (Hashem) was king over Yeshurun (Klal Yisrael) when the heads of the nation gathered together in unity with all the tribes of Israel" (V'zos Habracha 33:5.) Rashi explains that when Klal Yisrael join together in one group - b'agudah achas - and there is peace among them (like when the Torah was given), then Hashem is their king, but not if there is discord among them.
A king cannot reign without subjects (ein melech b'lo am.) The more focused the king's subjects are on serving the king and not on furthering their own personal interests, the more powerful is the king's rule. Thus, when Klal Yisrael join together in a unified service of Hakadosh Boruch Hu, they actually strengthen Hashem's monarchy. This is one of the goals of the tefilla of the yamim noraim - to declare our commitment to the kingship of the Ribbono Shel Olam, and to accept upon ourselves to bond together to strengthen that kingship.
On Sukkos, we gain the perspective needed to maintain that sense of unity. We are commanded to leave our homes and sit in the sukkah. Chazal explain (Sukkah 2a) that the Torah is telling us, "Leave your permanent home (diras keva) and sit in a temporary home (diras arai)." The flimsy, temporary physical structure of the sukkah serves as a reminder that our physical homes are really only temporary, and we should focus our attention on spiritual pursuits which have eternal value, not on the ephemeral, fleeting pleasures of the physical world (see Vilna Gaon to Sefer Yonah 4:5). The sukkah must have more shade than sun to symbolize that one's involvement in the material world should be secondary to one's preoccupation with ruchniyus. As the Mishna says (Avos 1:15), "Make the study of Torah your primary pursuit."
But the sukkah is also a symbol of peace and unity. Chazal say (Sukkah 27b) that one may fulfill the mitzvah of sukkah even with a borrowed sukkah because the posuk "Every citizen of Israel should dwell in Sukkos" (Emor 33:42) implies that all of Klal Yisrael can sit in one sukkah. The language that Chazal use that all of Klal Yisrael can sit in one sukkah highlights the idea that the sukkah is a structure that unifies the Jewish people.
How does the sukkah bring the Jewish people together? Rav Dessler (Michtav M'Eliyahu, vol. 2, p. 170) explains that the diras arai of the sukkah reorders our sense of priorities. It reminds us that our lives should revolve around avodas Hashem, rather than pursuing the transient pleasures of the material world. The more people focus on satisfying their physical desires, the less likely they are to be able to live together in peace and harmony because each one wants what the other has, and that leads to competition and disagreement (see also Rabbenu Yona, Sha'arei Teshuva 1:30). But when people concentrate on spiritual activities, and their sole desire is to serve the Ribbono Shel Olam, they join together with a unified sense of purpose.
Perhaps this is the connection between the yamim noraim and Sukkos. Yom Kippur is referred to as "a day to increase love and friendship, a day to abandon jealousy and competition" (tefillas mussaf of Yom Kippur). On Yom Kippur we abstain from certain physical activities, and we concentrate almost exclusively on tefillah and avodas Hashem, and that unified spiritual focus allows us to bond together. The sukkah teaches us that the way to stay united throughout the year is by maintaining our spiritual focus. The more involved we are in Torah and mitzvos, the more we will be able to join together in our service of the Ribbono Shel Olam, and that sense of unity will strengthen Hashem's kingship and add to His honor.
It is not surprising then that the Beis HaMikdash is referred to as a sukkah. In the tefillah of ma'ariv, we ask Hashem to "spread over us the sukkah of Your peace (ufros aleinu sukas shlomecha)". Similarly, at the end of birchas hamazon on Sukkos we add, "May the merciful One resurrect the fallen sukkah of Dovid (sukkas Dovid hanofales)." The Beis HaMikdash is the ultimate place of peace and unity. It can stand only when the hearts of Klal Yisrael are unified, not when there is disagreement and strife among them (Yoma 9b). Hashem will reveal the full glory of His Shechinah if Klal Yisrael is b'agudah achas. But that can happen only if we appreciate the message of the sukkah. When all of Klal Yisrael focus on developing their spiritual essence, they are able to rise above their insignificant differences and they unite in serving Hakadosh Boruch Hu.
This year, the message of the sukkah takes on new meaning. We might be physically distant from each other, but we can still bond together emotionally. When we focus on what is truly important in life, not only do we connect more with Hakadosh Boruch Hu, but we unite as a people and merit the ultimate bracha of ufros aleinu sukas shlomecha.