Succos

The Gr"a explains the reason we celebrate succos on the fifteenth day of tishri. Klal Yisroel lost the clouds of glory that escorted them out of Mitzraim on the 17th day of Tamuz- when they made the eigal. It didn't return to them until they did something solid- when they started building the Mishkon. Hashem granted us forgiveness on the tenth day of Tishri- Yom Kipper. The next day- 11th day of Tishri- Moshe told Klal Yisroel to build the Mishkon. The next two days-12th and 13th of Tishri- Klal Yisroel donated the material for the Mishkon. On the 14th day the gathered and took inventory of what was donated. The actual building of a house for Hashem began on the 15th day of Tishri- the day known to us as the first day of Succos. On that day the Clouds of Glory were given back to Klal Yisroel and they were granted final forgiveness.
R' Yitzchok Hutner points out that at the Simcha Bais Hashovaivah there was a special song that the Baali Teshuva sang. The whole Yom Tov comes as an extension- and gives a finality- to the Yommim Noraim. The Month of Tishri starts with the fear and trepidation of the Yemai Din. It closes with days of Simcha that we can get closer and that we were successful in getting closer to Hashem. That over the Ymai Din Hashem decided that Klal Yisroel is a Nation of eternity.
(The Medrish- Vaikra Rabbah 30;2- says that that is what the Lulov shows that we came out of the courtroom with our hands held high as a sign of victory.)

(I apologize about the grammar for it's getting late)