The last day of the holiday of Sukkot is called Shemini Atzeret. It is kind of a festive gathering because we have corrected the entire soul, the light has entered and filled it.
This ends the long period from the days preceding the New Year (Rosh HaShanah) to the last day of Sukkot.
Question: The name of the holiday “Shemini Atzeret” is taken from Tanakh. It is interesting that the celebration of Simchat Torah, which was introduced by the sages in the 6th – 7th century AD, falls on this day. What is the connection between them?
Answer: The state when we complete all corrections completely, receive seven lights, and they fill our soul, is called the Rejoicing of the Torah (Simchat Torah).
The Torah is the upper light that enters the corrected Kli (vessel) during the seven days of Sukkot, meaning the seven degrees. Then we celebrate the end of our corrections.
From KabTV’s “Spiritual States” 10/7/19