What was the original intent and meaning of Yom Shmini ha-Atzéret?

Shalom everyone, Yesterday at sunset began the day which rabbinic Judaism knows as ‘Simchat Torah‘ (‘Rejoicing of the Torah‘), but which the Torah itself refers to as Yom Shmini ha-Atzéret (Eighth Day of Closed Assembly) – you will soon find out that they are not...