Additionally, it is specifically on the 15th of the Hebrew month of Tishrei that the holiday of Sukkot begins because the 15th of the month is always indicative of the strongest expression of the energy of that month. Since the theme of the holidays of Tishrei is the return to God, reality, and one's true self, Sukkot—the holiday in which we live out this consciousness in action, with our physical dwelling places, with our physical objects, and with the "physical" people who personified these ideals—is celebrated from the 15th of Tishrei.
In this manner, the Hebrew month of Tishrei (which contains the holiday of Sukkot, as well as Rosh Hashonah and Yom Kippur) truly serves as the month of return—the return to God, return to reality, and the return to oneself—in a complete manner. Thus, the word TiShReI is spelled with the last letters of the Hebrew alphabet going backward (indicating a return) with the letter yud (indicating wholeness and spirituality since the letter yud has the shape of a mere point expressing existence that is beyond the physical, since a point exists on the page yet is not drawn out into the physical).
Wishing everyone a joyous and meaningful Sukkot!