ולקחתם לכם ביום הראשון "And you shall take for yourselves on the first day [the four species]" (Wayyiqra 23:40). The Midrash (Wayyiqra Rabba 30) tells us that "the first day" refers to the first day that our sins are calculated.
As we know, carrying the four species is a sign of our emergence meritoriously from the judgment on Yom Kippur. The way of the world is that when one emerges successfully from a court case, it is a cause for immediate celebration and the person feels he can now relax. How different are the Jewish people! After emerging successfully from Yom Kippur, they immediately set to work to perform more of G-d's beckoning and commandments. We are busy with building a Sukkah, selecting and buying the four species and generally preparing for the Holiday.
Not only do the Jewish people not sin at that time, but neither does G-d take note of their transgressions. The first day that G-d starts noting the sins of the Jewish people, is the 15th of Tashri. And what are the Jewish people doing then? They are busy doing Miswoth on the first day of Sukkoth.
(See Sefer HaToda'ah, Sukkoth)
Sukkoth, Sukkot, Succos