I heard during a shiur that the Yom Tov of Chanuka is the continuation and end of the days of Teshuva that began on Elul and Kapparah or forgiveness can still be reached until the end of Chanuka. Is that true?

Indeed Toldos Aharon (Miketz), Bnei Ysoschor, Tzemach Dovid (quoted by Iturei Torah p. 136). and others maintain that it is so. Some quote the posuk (Yeshayahu 23: 9) “Bezos Yechupar Avon Yaakov,” namely, that the sins of Yaakov will be redeemed on “Zos” or on the last day that carries the title of “Zos Chanuka.”