It’s not the Feast of Tabernacles already, so what’s the connection?
The non-answer is that it follows the seven days of Sukkot. That doesn’t entitle it to be called the Eighth Day. There are several connections.
To find out why Simchat Torah is called the Eighth Day, we first need to understand what the Festival of Booths is all about. The Torah tells us that the Festival of Booths comes to remind us of the 40 years that we lived in tents in the desert after our Exodus from Egypt. This desert trek stands for:
- Being provided for (Manna). The Festival of Tabernacles is also the Harvest Festival in which we show our gratitude.
- Being protected by G^d against all danger. By now living in huts, we show that we only trust in G^d’s Supervision and Protection.
- Being freed to Learn Torah and build good character traits.
- Each of the three above gives happiness. Sukkot is also called the Time of Our Happiness.
After the Feast of Tabernacles, how to take it into the rest of the year? Food is covered. Then, how to recreate our trust and happiness? By being happy with the Torah. Simchat Torah is the 365/24 message of Sukkot.
On a funny note: We read from the second Torah scroll the Creation Story, ending in what we say in the Friday night Kiddush, that G^d ceased work on the seventh day. The Rabbis teach us that G^d also keeps the Torah. The first line of the ensuing third scroll tells us we should not work on the eighth day. Don’t think that G^d discontinued working on Shabbat and then, on the eighth day, returned to creating the world. The Six Days of Creation was a one-time event. Ever since it’s only a little touching up.
At the end of the Torah, we read that no person knows where Moses’ grave is until this day. We know the Torah doesn’t use superfluous words. Then, why does it say ‘person’? Why not say that it’s not known? And why doesn’t it say it’s already not known? Why ‘until this day’?
We read that Moses’ grave was opposite Beit Peor (Deuteronomy 34:6), opposite Baal Peor, where the Jewish People sinned (Talmud, Sotah 14a) by connecting to the Idol Baal Peor (Numbers 25:3-5). Each year, when that date comes around, the Idol rises to prosecute and insult the Jews. But then, it sees Moses’ grave and sinks into the ground until its nose and stays silent (Tosafot to Sotah 14a). No person knows, but an idol does; not forever, but only until this day, when it rediscovers it, year after year.