The Children of Israel were commanded to leave their permanent homes and dwell in sukkot on the fifteenth day of the seventh month, to commemorate the sukkot which G‑d provided for them when they came out of Egypt. One might ask: Since the Exodus from Egypt took place in the month of Nisan, would it not be more appropriate to commemorate the event in Nisan? Why are we commanded to observe the Festival in the month of Tishrei?

While many answers have been given by our Sages, the essence is that the fulfillment of the mitzvah of sukkah should be recognized as being done for the sake of G‑d. During Nisan, in the spring, the weather becomes warmer, it is pleasant to dwell outdoors, and farmers often leave their houses for booths in their fields. During Tishrei, however, when the nights are cold and the rainy season is about to begin, farmers return to their homes. Thus, when Israel leave the comforts of their homes during Tishrei to dwell in sukkot, it is obvious to all that they do so to fulfill G‑d's commandment, for the sake of His Name. The Torah states: So that your descendants shall know (Leviticus 23:43) - the fulfillment of the mitzvah of sukkah should be known and recognizable as being for the sake of G‑d.

Our Sages also taught: Why do we make a sukkah after Yom Kippur? On Rosh Hashanah, G‑d sits in judgment over all the world's inhabitants, and on Yom Kippur, He seals the judgment. Perhaps it was decreed that Israel be exiled? They therefore build a sukkah and [voluntarily] exile themselves from their homes (Yalkut Shimoni 653).

The Later Authorities and commentators offered a number of additional reasons for commemorating Sukkot in the month of Tishrei rather than in Nisan:

It would not be fitting to commemorate the clouds of glory which surrounded the Children of Israel when they left Egypt, in the month of Nisan, for those clouds of glory were withdrawn after the episode of the golden calf. We commemorate only the clouds of glory which returned to them later [after Yom Kippur], for the second clouds remained with them during the entire forty years of their wanderings in the desert.

When the Children of Israel sinned during the episode of the golden calf, the clouds of glory withdrew from their encampment. Moshe ascended to Heaven three times. When he descended the third time, he brought Israel the mitzvah of constructing the Mishkan [Tabernacle] as a sign that G‑d had become reconciled with them and would dwell in their midst.

This took place on Yom Kippur. On the day after Moshe descended, Moshe assembled the entire congregation of Israel and Moses said: "Take from among you an offering for G‑d" (Exodus 35:1,4).

These offerings were brought on the 12th and 13th of Tishrei. On the 14th of the month, the "wise of heart" [the artisans who built the Mishkan] took the contributions from Moshe and on the 15th, they began the construction of the Mishkan. It was then that the clouds of glory returned and became a sukkah, a protective shelter over the camp of Israel.

Accordingly, that day - the 15th of Tishrei - was fixed for the people as the time when they were to dwell in sukkot. Just as G‑d left Heaven, as it were, and caused His presence to dwell on earth in the Mishkan in the midst of the camp of Israel, so too does Israel show G‑d that they also leave their homes and dwell with Him in a sukkah, in the protective shelter of His faithfulness.

Another reason: The Festival of Sukkot is celebrated at the harvest season, when people gather the produce of the land and bring it into their homes. So that they do not become arrogant and find all their happiness in material possessions, the Jewish People are enjoined to leave their homes and possessions and to go out to the sukkah, protected only by the wings of the Divine presence, thereby declaring: The earth and all that it includes is G‑d's (Psalms 24:1). The only true happiness is in G‑d - in the shelter of His Shechinah.

It is for this reason that it is customary - according to the Ashkenazic tradition - to read the book of Ecclesiastes on Sukkot, so that we might temper our pride in our possessions and realize that all is vanity (Ecclesiastes 1:2). To what then should we direct our thoughts? To the concluding verse of Ecclesiastes (ibid. 12:13): The sum of the matter is that when all has been considered, [one must conclude that] G‑d is to be feared and His mitzvot are to be fulfilled, for this is the whole [purpose] of man.

Another reason: during the Days of Awe, all Israel repent for their sins committed during the entire year. Although their repentance is accepted and their sins are pardoned, they nevertheless remain troubled because of their earlier sins. 

Penitents often feel as if they can find no place for themselves in the world. G‑d therefore tells them: "Since you can find no place of rest in the world because of the shame you feel, I will make you a place. Come to Me and find protection in My shelter - in the sukkah of My peace."

As In the Days When We Left Egypt

Every citizen in Israel shall dwell in sukkot, so that your descendants shall know that in sukkot I caused the Children of Israel to dwell when I brought them out of the land of Egypt (Leviticus, 23:42,43).

When Israel left Egypt, they left with a wealth of possessions - with the spoils that they had taken from the Egyptians and with the wealth which they had recovered from the sea. They came to the wilderness and found no vegetation, no water, and no dwelling place. What pleasure could they derive from all their possessions? G‑d said to them:

"Take your silver and gold, and all your other precious objects, and store them away. You shall eat bread provided from Heaven and drink water from the rock. Your clothing will not wear out, nor your shoes upon your feet. All of your sustenance shall be from Me. And if you say, 'What use is our great wealth to us?' [I say:] 'Give it for the work of the Mishkan'...and I shall consider it as if you had sustained Me with your wealth."

When Israel arrived at their "place of rest and inheritance" [i.e., the Land of Israel], after they had toiled in the fields and vineyards and their Land had yielded its produce, upon gathering their bounty they would leave their houses and dwell in temporary booths. So that your descendants shall know that in sukkot I caused the Children of Israel to dwell - the bounty which had been gathered was not merely the fruit of the land, for the blessing of G‑d is that which brings wealth. [G‑d says:] Just as I have given you blessing, so too shall you, My children, bless Me with what I have given you. And you shall celebrate it as a Festival for seven days a year (ibid. 23:39).

Now, in the case of the Festival of Pesach, the verse does not state seven days a year, but rather seven days, whereas, when referring to the Festival of Sukkos, the Torah states: seven days a year. The difference in expression comes to teach us that these seven days of rejoicing before G‑d through the performance of His mitzvos and dwelling in the sukkah are regarded as if Israel had rejoiced before G‑d all year. All of their work during the entire year is considered to have been only for the sake of G‑d’s glory.