Sukkot is also called the Festival of Ingathering – “the Festival of Ingathering at the turn of the year” (Shemot 34:22) – because it occurs when the grain and other crops are gathered in from the fields and brought into homes and storehouses. Thus, the verse states: “Hold a seven-day festival of Sukkot when you gather in from your threshing floor and your winery” (Devarim 16:13, and similarly Vayikra 23:39).
The three pilgrimage festivals are connected to the agricultural seasons during which they occur, as the Torah states:
Three times a year you shall hold a festival for Me: the Festival of Unleavened Bread…at the ordained time of the month of Aviv…. The Festival of the Harvest of the first fruits of your work, of what you sow in the field; and the Festival of Ingathering at the end of the year, when you gather your handiwork in from the field. (Shemot 23:14-16)
Pesaḥ is in aviv (spring), when everything is beginning to bloom; Shavu’ot is at the end of the grain harvest and the beginning of the fruit harvest; and Sukkot is at the completion of the ingathering of the year’s yield. The mitzva of each festival is to rejoice and thank God for the bounty with which He has blessed us. Sukkot, when we finish gathering in the produce of the whole year, is therefore the most joyful of all (Peninei Halakha: Mo’adim 1:2; 13:4-5).
These natural processes of this world reflect the spiritual processes that occur in the supernal worlds. Pesaḥ is a time of beginning and renewal, so we left Egypt then and became a nation. Shavu’ot is a time when the growth process reaches maturity, so we received the Torah then. Sukkot is when we finish gathering the grain and fruit into the house, so spiritually, it is a time to collect the spiritual fruits that Israel gained during the Egyptian bondage and the wanderings in the desert – and bring them into the home, that is, into Eretz Yisrael, Israel’s home. As Abarbanel wrote (Devarim 16:13), the primary joy of Sukkot is inheriting the land of Israel. Thus, Pesaḥ celebrates the uniqueness of Israel revealed at the time of the Exodus; Shavu’ot celebrates the giving of the Torah; and Sukkot celebrates inheriting Eretz Yisrael.
Two cycles culminate with Sukkot. The longer cycle is that of the three pilgrimage festivals, which correspond to the agricultural cycles and seasons, and which will begin anew with the winter planting. The second cycle is the repentance and atonement that we experience during the months of Elul and Tishrei. We do many wonderful things in the course of the year, but by our very nature, we are also prone to sin. In order to complete the year on the most positive note possible, we must repent, cleanse, and purify ourselves from any evil still clinging to us. This is our spiritual undertaking during Elul, Rosh Ha-shana, the Ten Days of Repentance, and Yom Kippur. By virtue of this repentance, atonement, and purification, the good we have absorbed during the course of the year is further refined, cleansed of the evil that has clung to it. This enables us to multiply our joy on Sukkot.
Rav Kook explains that although repentance is tremendously important, as it cleanses hearts and purifies disgraceful actions, it is also accompanied by pain, which causes the dulling of good will and saps vitality. Therefore, the season of repentance culminates with the joy of Sukkot, which restores our will to do good and our bold vitality (Orot Ha-teshuva 9:10).