The holiday of Sukkos is unique in that it is the last holiday of the Jewish year (we count the new year as starting from Tishrei, as we regard Tishrei as the month during which man was created. We count holidays from the month of Nissan).
Not only is it the last holiday of the year, it is also the culmination of two series of holidays. It is the culmination of both of the “Shalosh Regalim”, the three major holidays when one is required to go to the Temple — Pesach (Passover), Shavuos, and Sukkos — and of the “Yomim Norayim”, the days of awe — Rosh Hashanah and Yom Kippur — which are followed by Sukkos.
Let us first explain how these two series of holidays culminate in Sukkos.
Sukkos is the third leg of Pesach and Shavuos. Pesach is compared to an engagement, when we first encountered G-d. Shavuos is considered the marriage, when we entered into a covenant with Him by receiving the Torah. Sukkos is the honeymoon after the marriage, when we go “live” with G-d.
Sukkos is the culmination of Rosh Hashanah and Yom Kippur, by virtue of the clarity achieved on Sukkos. Sukkos is called zman simchaseinu — season of happiness. On Rosh Hashanah we reinforce the concept that G-d is King. Once we know this, we are able to truly beseech G-d for His forgiveness on Yom Kippur. Having achieved that, we reach a point of clarity — happiness on Sukkos, and to express this happiness and clarity, we leave the security of our homes and move into a dwelling with G-d — the Sukkah.
Sukkos, the last Biblically ordained holiday of the year (Chanuka and Purim are Rabbinical holidays), also teaches us an important lesson.
Each holiday comes to sanctify a specific human expression. Pesach sanctifies our mouths through the eating of matza. Shavuos sanctifies our intellect through accepting of the Torah. Rosh Hashanah sanctifies our ears through listening to the sound of the shofar (the commandment on Rosh Hashanah is to hear the shofar).
The commandment to see the Temple on Peasch, Shavuos and Sukkos sanctifies the eyes — “Three times you shall appear before G-d” (Deut.16:16). Sadly, due to the loss of the Temple, we are unable to properly perform this commandment.
Yom Kippur, which is called Shabbos Shabbason, a double Shabbos, is when we focus on our neshama (soul) and strictly spiritual activity (thus no eating, drinking, wearing of shoes, etc.). On Sukkos itself we have the commandment to take the four species (lulav, esrog, aravah, and hadas) which sanctifies our hands and sense of touch.*
On Sukkos, we have the unique privilege of immersing our entire body in the mitzva by living in the Sukkah (one must eat meals and sleep in the Sukkah). It is the only commandment we do which involves our entire body (Although Sukkos also has the mitzva of the four species, as mentioned above, the definition of a holiday is found in its name).
When we observe all the holidays we are able to sanctify our physical and spiritual selves in many ways.
* The only sense not mentioned is the sense of smell. The sense of smell is the only sense not used by Adom (the first man) when he sinned in the Garden of Eden. In fact, the sense of smell is used during the havdalah service that marks the departure of Shabbos, when we smell spices to restore our soul.