Only on the first day of Sukkot, when we fulfill the commandments of Sukkah and Lulav, do worldly matters “settle in” and take hold with the desired level of sanctity. Therefore, the fifteenth of Tishrei is the first day for reckoning transgressions.
Some days possess special intensity. Such days, if we are worthy, turn into holidays. If, Heaven forbid, we are not worthy, they can become days of extreme trial - as in the time of Gedaliah ben Achikam. But the possibility of rectification remains.
The days between Yom Kippur and Succoth are directed towards preparing man for his return to the affairs of this world in a suitable and sacred manner. The transition involved in descending from the height attained on Yom Kippur to normal daily existence.