None of the festivals is as rich in mitzvot as is the Festival of Sukkot. Its mitzvot include: the Torah obligations of building a sukkah and dwelling in it; the four species; the water libation ceremony, which is a law which Moses transmitted but which is not recorded in the Torah); the taking of the aravah (willow) on Hoshana Rabbah, which is a custom that dates from the time of the Prophets; and the specific commandment to rejoice: And you shall rejoice in your Festival (Deuteronomy 16:14).
Why does Sukkot have so many mitzvot, right after Rosh Hashanah and Yom Kippur? R. Avin explained: To what can the matter be compared? To two men who were called before a judge and we have no idea which one of them won the case. (How can we determine who has prevailed?) The one who carries the scepter in his hand (when he leaves the judge's chambers) is the one who has prevailed.
Similarly, Israel and the nations come and contend before G-d on Rosh Hashanah, and we do not know who has prevailed. However, when Israel emerges from G-d's presence with lulav and esrog in hand, we know that Israel has been victorious (Leviticus 30).
What comprises Israel's victory? They emerge from Divine judgment meritorious, without guilt, and they are forgiven for all of the sins committed during the year. They remain G-d's beloved children and are loved by Him as before. They carry the King's sceptre, fulfilling His commandments joyously.
They dwell in the sukkah, take the four species, draw water in joy, pour the water libation on the altar as commanded, take the aravah on Hoshana Rabbah, offer the sacrifices and burnt offerings to fulfill the will of their Maker, bringing atonement for themselves and for all of humanity. And when the seven Festival days have ended, they do not hasten to return to their homes, but rather they tarry and assemble before their King and celebrate an eighth day rejoicing over His Torah (the Festival of Simchas Torah).
Our Sages explain the words in the verse describing the four species as follows: And you shall take for yourselves on the first day (Leviticus 23:40). The first day - the first day on which Israel's sins are (again) calculated. Usually, when someone wins a trial, he feels relieved and happy and immediately wants to celebrate and relax. Israel, however, is different. After having emerged meritorious from Divine judgment on Yom Kippur, they do not turn to rejoicing but rather occupy themselves immediately with the fulfillment of G-d's mitzvot, not celebrating until Sukkot begins, on the fifteenth day of the month (Tanchuma, parashat Emor).
During the four days between Yom Kippur and Sukkot they are busy constructing their Sukkot and acquiring the four species, and they do not come to sin. G-d therefore does not engage, as it were, in recording their sins on those days, since they are completely occupied with fulfilling His commandments. Which day then is "the first day on which Israel's sins are calculated"? It is the fifteenth of Tishrei, the Festival of Sukkot, a day which is replete with mitzvot as a pomegranate (is filled with seeds).
And you shall take for yourselves on the first day the fruit of a beautiful tree (Leviticus 23:40)
Since this refers to the fifteenth day, why then is it referred to as "the first day"?
This can be compared to a province whose citizens owed a fine to their king. He set out to collect it, and when he was still ten mil from the province, the dignitaries came out and offered him praise. The king promptly reduced their debt by a third. When he came to within five mil of the province, the minor officials came out and offered him praise; the king promptly reduced the debt by another third. When the king entered the province itself, all of the residents - men, women, and children - came out and praised him. What did the king do? He promptly remitted the entire amount due him, and then he declared: "What has passed has passed. From this point on, we shall begin a new accounting."
Similarly, on the eve of Rosh Hashanah, the greatest men of the generation fast and, we are told, G-d reduces Israel's sins by a third.
During the period between Rosh Hashanah and Yom Kippur some individuals fast and it is said that G-d reduces Israel's sins by another third. On Yom Kippur, all Israel fasts. G-d therefore tells them: "What has passed has passed. From this point on, we shall begin a new accounting."
From Yom Kippur until Sukkot, the entire nation is engaged in the fulfillment of the mitzvot. One is busy with his sukkah, the other with his lulav. On the first day of the Festival of Sukkot, all Israel stands before G-d with their lulavim and etrogim in hand, to glorify Him. G-d then says to them: "What has passed has passed. From this point on, we shall begin a new accounting."
Therefore, Moses told them: And you shall take for yourselves on the first day. That is, for yourselves - "for your own benefit". On the first day when sins are again calculated, busy yourselves with mitzvot so that your accusers will become your defenders.