Based on a Naaleh.com shiur by Mrs. Shoshie Nissenbaum
One would think Sukkot should have been after Pesach, when Hashem took us out of Egypt. That was when the Jews dwelt in sukkot in the desert. Yet the holiday comes close on the heels of Rosh Hashana and Yom Kippur. It is as if Hashem says, “You invited me into your home, now I will invite you into my abode.”
Sukkot contains an aspect of the world to come. For one special week we merit to dwell in the shade of the Divine Presence. The halachot (laws) of this special mitzva help us understand how to come closer to Him. Everything in the physical world has a form and shape, something that gives it borders. Holiness, however has no boundaries. Just as Hashem is expansive and fills the world, spirituality has no limits. The sukkah‘s width is boundless. This teaches us that everything in the world can be included within the framework of kedusha (sanctity). We sleep and eat and spend the greater part of our time in the sukkah as a way of showing Hashem that all physicality can be sanctified for Him. Yet the walls of the sukkah cannot be higher than twenty amot because the boundaries of kedusha require a vessel.
The Ramchal in Mesilat Yesharim writes that a person can make himself into a mishkan (tabernacle) for Hashem. Just as the mishkan traveled from place to place, a person can connect to Hashem wherever he is. The more a person attaches himself to Hashem, the more he transforms himself into a dwelling place for Him. On Sukkot we take everything we have and place it within the firm boundaries of the sukkah walls and elevate it for Hashem.
Sukkot comes after Rosh Hashana and Yom Kippur, days of tremendous closeness to Hashem. On Rosh Hashana we pray for sustenance, life, good health, children and a sweet new year. The sweetness is the aspect of uplifting what we have for Hashem. On Sukkot we actualize this by inviting Hashem into our homes and hearts.
The Gemara says that the merit of building the walls of the sukkah drives away both our physical and spiritual enemies. The sukkah protects us. It must have more shade than sun. Sun represents the power of the nations. It never changes or grows. We are compared to the moon, which constantly experiences renewal and rebirth.
Sukkot is a tremendous opportunity to store up kedusha and tahara (purity). This is why it is called zman simchateinu. This is what eternal joy is about.