....Of course, the Arba Minim [the Four Species] represent 4 categories of Jews:
1) The etrog tree, which possesses taste and scent, represents the masters of Torah and mitzvah, who are complete tzaddikim
2) And the willows, which possess neither taste nor scent, represent those who possess within themselves neither "taste" [ta'am=purpose/taste] nor scent, which symbolizes the groups that possess neither Torah nor mitzvah. And they are complete rashaim.
3) And the date-palm fronds [lulav] which give forth food, but not scent
4) And the branch of braided tree [myrtle] which gives forth scent, but not food
—both of them [the last two] represent the benonim, who either possess Torah or mitzvah.
1) At first, He waives the transgressions of the groups symbolized by the etrog. And they are the spiritual giants who are the first to welcome the Blessed One on Erev Rosh Hashanah.
2) Later, He concedes one-third for the benonim that welcome him during the Ten Days of Repentance, and who are symbolized by the date-palm fronds [lulav] and the branch of braided tree [myrtle].
3) And then later, on Yom Kippur, He concedes to everyone, even the rashaim, who are symbolized by the willows because everyone welcomes Him and are fasting [in repentance].
This is why the Kohen Gadol could only come to the Kodesh Kadoshim on Yom Kippur in a cloud of Ketoret that contained a mixture of chelbanah [the bitter galbanum] with the Ketoret Spices—to include all the sinners of Yisrael and to make them as one unit [agudah achat] with the good ones in order for these [the good Jews] to atone for those [the sinful Jews].
To annihilate them is impossible.
Instead, they will become one unit and these will atone for those....
Therefore, the Kli Yakar explains that Hashem orchestrated the spiritual mechanics to enable individual teshuvah to be calculated as mass teshuvah in the Heavenly accounting. And He does so by melding the different categories of Jews into one single unit, starting on Rosh Hashanah.
But why is it actualized on Sukkot?
Because, explains the Kli Yakar, the ten-day period that spans from Rosh Hashanah until Yom Kippur transforms individual teshuvah to the status of mass teshuvah.
And from Yom Kippur until Sukkot, all Jews are (theoretically, anyway) engrossed in the myriad mitzvot involving preparations for Sukkot (like obtaining the Arba Minim and building a sukkah, and meal preparation, etc.).
This constant mitzvah-involvement protects the Jewish people during that time.
The Kli Yakar explains:
However, the first day of the [Sukkot] holiday is the first for the accounting of transgressions, therefore, the establishment of mass teshuvah needs to be done on that very day. And when all four of these groups will be as one unit, these [the good Jews] will atone for those [the sinful Jews].
So the ability to have our teshuvah count for maximum effect now, during the time between the Spring holiday of Pesach and the Summer holiday of Shavuot, goes all the way back to the heavy Autumn season of Rosh Hashanah, Yom Kippur, and Sukkot.
It just shows how much Hashem really wants things to work out for us.
This is my own translation and any errors are also mine.