The Torah, “Leviticus,” 23:33 – 23:36: And the LORD spoke unto Moses, saying: Speak unto the children of Israel, saying: On the fifteenth day of this seventh month is the feast of tabernacles for seven days unto the LORD. On the first day shall be a holy convocation; ye shall do no manner of servile work.
Seven days ye shall bring an offering made by fire unto the LORD; on the eighth day shall be a holy convocation unto you; and ye shall bring an offering made by fire unto the LORD; it is a day of solemn assembly; ye shall do no manner of servile work.
The Sukkot holiday continues for seven days; in each one of them an ascent is carried out to the spiritual levels of Hesed, Gevura, Tifferet, Netzach, Hod, Yesod, and Malchut.
And on the eighth day a special gathering happens when a person receives filling by the Light of Hassadim (Mercy), the light of love, friendship, happiness, compassion, partnership, and mercy. This is called “embracing from the right side,” as it is said: “His left hand should be under my head, and his right hand should embrace me” (Song of Songs 8:3).
This means that Rosh HaShanah and Yom Kippur are “His left hand should be under my head,” and Sukkot is “and his right hand should embrace me.” And on the last day, on the eighth day of the Sukkot holiday, a partial coupling of the male and female parts of the Kli happens.
At this time, the desire of the person transmits illumination through the Upper Light, and so throughout all the days of the festival, the revelation of the greatness of the Light of Hassadim and great adhesion of the male and female parts of the desire to “embrace” one are expected.
So every day of the festival of Sukkot is important, those that are spent in a Sukkah that is specially built under the dome of heaven where a sacred gathering and feast are arranged.
A person who is in a Sukkah for seven days seemingly receives all the corrected Ohrot Ha Makifim (Surrounding Lights), filling and pushing him toward discovery, connection, and contact with the Creator.
And this happens on the eighth day upon leaving the Sukkah, so the eighth day is called Simcha Torah. Torah is the Light that corrects the person and fills him with happiness.
Comment: It is thought that every evening members of the Ushpizin (guests), visitors, arrive at the Sukkah according to the order: Abraham, Isaac, Jacob, Moses, Aaron, Joseph, and David.
Answer: All of these visitors symbolize the Lights of correction in seven spiritual levels. They are not historical figures but spiritual states. Each one of the levels possesses a unique character, and only when a person passes through them, meaning that he corrects himself in seven different states, does he become ready for the revelation of the Creator.
From KabTV’s “Secrets of the Eternal Book” 6/18/15