Teshuva With Peace of Mind:
The pasuk states (Vayikroh 23:42): "You shall dwell (teishvu) in sukkos for seven days." Rav Yaakov Dovid of Radmosk zy"a (Sefer Avnei Hamakom) quotes Rav Dov Berish of Biala zy"a as translating the word "teishvu" to mean "teshuva", and also to connote "yishuv hada'as" (peace of mind). He says that in actuality they are both the same thing, as to do teshuva, one needs yishuv hada'as.
Chag Ha'asif:
The pasuk states (Shemos 34:22): "And the Chag Ha'asif (festival of the ingathering of the crops), the period of the year." Sefer Toras Avos quotes Rav Mordechai of Lechovitch zy"a as explaining this to mean that through the influence of this yomtov, one can gather the strength to serve Hashem properly all year long.