The name used in the Torah for the month of Tishrei is the Seventh Month, HaḤodesh HaShevi’i. The Sages interpreted the name homiletically and said that the month is replete with mitzvot, where the word for replete, mesuba, is from the same root as hashevi’i (Vayikra Rabba 29). More specifically, they characterized Sukkot as a Festival whose mitzvot are abundant. This wealth of mitzvot is manifest in the Torah in the numerous verses devoted to the halakhot governing the Festival. However, beyond the mitzvot explicitly mentioned in the Torah, there are additional mitzvot that were received as halakhot transmitted to Moses from Mount Sinai, and they too impart a unique character to the Festival.
The festival of Sukkot is multifaceted, and its halakhot are not all associated with only one central theme; rather, there are several themes that differ from those of all other Festivals. In addition to the general mitzvot of a Festival, detailed in Massekhet Beitza, and the mitzvot of the pilgrim Festivals, detailed in Massekhet Hagiga, the festival of Sukkot is characterized by its distinctive mitzvot: The mitzva of sukka, which entails moving one’s residence and dining area; the special ceremony that involves taking the four species; the plethora of additional offerings unique to Sukkot; and the mitzvot and special ceremonies performed in the Temple.
In essence, the festival of Sukkot does not commemorate a specific historical event, in contrast to Passover, which commemorates the Exodus, and Shavuot, which commemorates the revelation at Sinai. Nor does it correspond to a specific stage in the agricultural cycle, unlike Passover, when the omer offering is sacrificed from the new barley crop, and Shavuot, when the offering of the two loaves is sacrificed from the new wheat crop. Rather, it commemorates the entirety of the experience of the children of Israel in the desert, the life of a people without a country, without claim to land, and without stability. This commemoration includes gratitude for all the miracles of the Exodus that transpired in the past, the clouds of glory, and the contrast between the life of a people wandering in the wilderness and the conclusion of the harvest of the produce of the fields and the trees in Eretz Yisrael. The four species, too, serve on the one hand as a kind of victory procession, celebrating past accomplishments (Vayikra Rabba 30); and on the other hand taking the four species includes saying a prayer for rainfall and a successful crop in the coming year, the very essence of the mitzva.
Beyond the rejoicing that characterizes all the Festivals, there is a special mitzva of rejoicing on Sukkot, to the extent that the name of the Festival coined by the Sages in the prayers recited during the Festival is, “The Time of Our Rejoicing.” As an extension of the thanksgiving and rejoicing, there are the special mitzvot received as halakhot transmitted to Moses from Sinai, which include mention of and prayer for rain and blessing for the coming year. The mitzva of the libation of water on the altar, a central component of the rejoicing of the Festival in the Temple; the mitzva of surrounding the altar with willow branches; and the waving of the willow branch on the seventh day of the Festival all evoke and inspire prayer for rain during the coming year. These are not explicit prayers and are certainly not accompanied by fasting and pleading. Rather, rain is a theme that arises in the course of the great rejoicing.
Although there are some mitzvot associated with Sukkot totally independent of the Temple, e.g., the mitzva of sukka, and there are others exclusive to the Temple, e.g., the additional offerings, there are many mitzvot associated with Sukkot in which there is a special connection between the mitzva and the Temple. Although they are mitzvot whose practice is not limited to the Temple, there is a difference between the practice in the Temple and the practice outside the Temple, e.g., the four species. The problematic nature of these mitzvot while the Temple was standing was exacerbated after its destruction, which severed the connection between the mitzva and the Temple. This led the Sages to institute ordinances to commemorate the Temple, which involved the introduction of many new halakhot.
These and other issues will be taken up in detail in Massekhet Sukka.