Tishrei is a comprehensive month, which provides us with spiritual resources and strength for a good year in all aspects of Judaism.
Therefore the holidays of the month contain al the elements of a proper, complete year: Rosh Hashanah, the day of G-d's coronation, provides awe of Heaven for the whole year. Yom Kippur provides repentance, and Sukkot and Shemini Atzeret and Simchat Torah provide the joy of a mitzvah, so that in the coming year all matters of Torah and mitzvot may be done with joy.
Similar to a visit to the market, where we procure all sorts of merchandise -- The month of Tishrei, enriches us with an abundance of experiences, so that we may draw sustenance from it all year long. We just need to unpack the merchandise and put it to use.
Likkutei Sichot, vol. 9, p. vii / vol. 14 p.442