The Torah states that the holiday of Sukkot is to be observed on “The fifteenth day of the seventh month (Tishrei)” (VaYikra 23:34). The Torah later explains the reason for the most iconic Mitzvah of the holiday, sitting in the Sukkah, as a commemoration for the “Sukkot” in which Hashem placed Bnei Yisrael after their exodus from Mitzrayim (23:43). As to what these “Sukkot” really were, Rabi Eliezer understands them to be the Ananei HaKavod that Hashem placed around the Jewish people (Sukkah 11b). The obvious question, then, is why the date of this holiday is not in Nisan, when the actual redemption took place and when Hashem first provided Bnei Yisrael with the Ananei HaKavod.
The most famous answer as to Sukkot’s placement in Tishrei is to make sure that we are doing the mitzvah LeSheim Shamayim, and not for our own benefit. Nisan falls out during the spring when the weather starts to warm up after a long winter, and people spend more time outdoors for their personal enjoyment. The Torah commands that we sit in the Sukkah during Tishrei – in autumn, when the weather is not as pleasant – to fulfill the command, “Lema’an Yeid’u Doroteichem,” “So that your [future] generations may know (that Hashem placed Bnei Yisrael in Sukkot in the desert)” (VaYikra 23:43). The fulfillment of the mitzvah has to be known and recognizable to all as being LeSheim Shamayim, and not for personal enjoyment.
An alternative answer is given by the Yalkut Shim’oni. It states that Sukkot is placed right after Rosh HaShanah and Yom Kippur because there may have been an exile decreed in Heaven during these days of judgment. The holiday of Sukkot then serves as a voluntarily exile for us (Yalkut Shim’oni 653). This voluntary exile will at least partly fulfill the decree made in Heaven and will hopefully lessen the harshness of the impending exile.
Some offer a simpler, yet brilliant, answer based on the Vilna Gaon’s commentary to Shir HaShirim 1:4. It would be completely useless to celebrate Sukkot in Nisan because the Ananei HaKavod left Bnei Yisrael shortly after they first arrived, once Bnei Yisrael made the Eigel HaZahav. We celebrate Sukkot in Tishrei because that was when the Ananei HaKavod returned. How do we know this? The sin of the Eigel HaZahav was pardoned by Hashem on Yom Kippur – the 10th of Tishrei – and the day afterward, the construction of the Mishkan was commenced. The materials were gathered on the 12th and 13th Elul, the builders received the materials on the 14th, and the actual construction of the Mishkan began on the 15th. On this day the Ananei HaKavod returned and the day was set as the time when Bnei Yisrael would sit in their Sukkot to remember this joyous occasion.
The fact that Sukkot is Chag HaAsif (Festival of Gathering) also plays a role in the timing of the holiday. It is during this time that farmers harvest their crops and bring them inside their homes. To ensure that no one would grow arrogant and think he is solely responsible for his harvest, we are all commanded to leave our homes into the shelter of Hashem. We humble ourselves by remembering that everything is in accordance with His will and that the only true happiness is in His Shechinah.
Rav Shimshon Refa’el Hirsch explained the timing of Sukkot to be a conclusion to the High Holiday period. He told a parable to explain this: There once was a king who had only one daughter, and when she came to be of marriageable age, he wanted to make sure that she would have the best husband. The king searched far and wide until at last he found the perfect prince for his daughter. The king finally married off his daughter, and no sooner that he turned his back did the prince begin to mistreat his wife. This continued for quite some time until word came to the prince that his father-in-law would be visiting. He began to improve himself and tried to make amends, and by the time the king came the prince was perfect again. The king stayed for several days and finally, right when he was about to leave, his daughter came running to her father and informed him of all the misdeeds the prince had done to her. The king was flabbergasted, but known for his mercy, he made a deal with the prince. The king said that his son-in-law could live with him until he learned appropriate behavior.
Similarly, Hashem has entrusted each and every one of us with a Neshamah (His daughter), but we tend to misuse it and treat it cruelly. During the month of Tishrei, Hashem comes for a visit and we improve ourselves a month in advance. He stays until the end of Yom Kippur, but at the end of Ne’ilah, just as He is about to leave, our Neshamot inform him of all the terrible things we have done. It is at this time that we “move in with Hashem” in the Sukkot, in order to learn how to improve ourselves permanently and hopefully maintain all our changes and resolutions for the new year.