Booths - According to Jewish tradition, what were used at the Feast of Tabernacles were strictly "tabernacula," structures of boards, with a covering of boughs.
The "booth" in which the Israelite kept the Feast, and the "tent" which was his ordinary abode in the wilderness, had this in common - they were temporary places of sojourn, they belonged to camp-life. The seven days of abode in the booths of the festival was thus a fair symbol of the forty years of abode in tents in the wilderness. The Feast might well become the appointed memorial of this period of their history for the ages to come.
All that are Israelites born - The omission of the foreigners in this command is remarkable. Perhaps the intention was that on this joyous occasion they were to be hospitably entertained as guests. Compare Deuteronomy 16:14.

Ye shall dwell in booths seven days,.... So that it seems they were not obliged to dwell in them on the eighth day, which was an holy convocation, a sabbath in which no servile work was to be done as the first, Leviticus 23:36. The eighth day was a day by itself, a sort of an appendage to the feast of tabernacles, when they went into their houses again, and kept it as an holy day; and perhaps principally in giving thanks for the ingathering of the fruits of the earth, to which this seems to be appropriated from Leviticus 23:39. According to the Jewish writers, they did not go out of their booths until they had dined in them on this day; and as they went out used to say,"may it be the will of God that we may be worthy the next year to dwell in the booth of Leviathan (c);''that is, to feast with the Messiah in the world to come. And to those days the Jews have added a ninth, which they call "the joy of the law", and which they keep for joy of having finished the reading of the law; which being divided into as many sections or lessons as weeks in the year, were so ordered to be read as to be finished at this time (d):
all that are Israelites born shall dwell in booths; the Targum of Jonathan is,"all the males in Israel, and even the little ones, that do not need their mothers, sit in the shades blessing their Creator, when they enter there.''And, according to the Misnah (e), women, servants, and little ones, are free from the booths (i.e. are not obliged to dwelt in one), but a little one, who hath no need of its mother, is obliged to dwell in the booths: and elsewhere it is said, that sick persons, and such as wait upon them, are not obliged, nor messengers upon any business, nor travellers and watchmen in cities, and keepers of gardens and orchards; if such travel, or keep watch in the day, they are obliged to be in them at night, and if in the night, then they are to dwell in them in the day (f). Jarchi says, that everyone born in Israel comprehends proselytes, who were bound by this law.
(c) Lebush, par. 2. c. 668. sect. 5. (d) Buxtorf. Synagog. Jude. c. 27. Leo Moden's History of the Rites of the Jews, par. 3. c. 7. sect. 6. (e) Misn. Succah, c. 2. sect. 6. (f) R. Alphes, par. 1. Succah, c. 2. fol. 374. 2. 375. 1.

In booths - Which were erected in their cities or towns, either in their streets, or gardens, or the tops of their houses. These were made flat, and therefore were fit for the use.