Sukkot is a complex time of the year. Within the holiday season, it is at the end of the season of the regalim, which starts with Pesach, in the first month of the Jewish calendar (Tishrei is the seventh, not the first month). In the agricultural year, which is so central to the calendar and the holidays, as described by the Torah (see Shemot 34:22), Sukkot is at (/near) the end of the harvest season, i.e., the end of the agricultural year. Therefore, at least if one has an optimistic perspective, this is a time of celebration, and we call it “z’man simchatenu.” This is a stark contrast to the mood of the recent Yamim Nora’im, which focus on fear and trepidation. Many explain Sukkot as a natural return to a more optimistic tone after hopefully succeeding in the spiritual “heavy lifting” of Yom Kippur.
The Vilna Gaon famously pointed out that there are two mitzvot that one fulfills with his whole body – living in Israel and living in a sukka. Of course this can be a cute physical observation, that there is no specific part of the body to which the mitzva applies. However, it is more likely that the intention is that these mitzvot apply to one’s whole persona.
“They shall live in the sukkot for seven days; every citizen in Israel shall live in the sukkot. So that your generations will know that in the sukkot I had Bnei Yisrael live when I took them out of the Land of Egypt; I am Hashem, your G-d” (Vayikra 23:42-43). In the midrash (Sifra, Emor 12) we find three explanations for this mitzva. There is an opinion that the sukka is to be made from the four species which are waved on Sukkot, which implies that it is essentially part of one unit with the mitzva of the four species. It is difficult, though, to divorce the mitzva from the historical context to which the p’sukim refer explicitly.