As far as I can tell every single Jewish Holiday celebrates a particular event that happened on the date we celebrate it. Namely:
Rosh Hashanah - (Tishrei 1) Creation of Man
Yom Kippur - (Tishrei 10) Final time Moshe decends Mt. Sinai
Sukkot - (Tishrei 15) ???
Chanukah - (Kislev 25) Miracle of the Oil
Purim - (Adar 14) Jew were saved from Hamans decree.
Pesach - (Nissan 15) - Jews left Egypt.
Shavuot - Jews received the Torah.
It is true that Sukkot commemorates the Clouds of Glory which protected the Jews in the desert, but I have not seen any source that this began on the 15th of Tishrei. Can anyone shed some light on this?