Halakha Imamat 23:34: Mishneh Torah, Shofar, Sukkah and Lulav & Arukh HaShulchan

[In a case of] one who did not make a sukkah (before the festival) — whether inadvertently or volitionally — he must make a sukkah during the intermediate days of the festival. He may even make a sukkah at the end of the seventh day — for its commandment is all seven [days]. The wood of the sukkah — whether the wood of the panels or whether the wood of the skhakh — is forbidden all eight days of the festival. We may not benefit from them for something else all eight days, because the sukkah is designated (muktseh) all of the seventh day until twilight — and since it is [also] designated at twilight of the eighth day (the twilight being the transition from the seventh day to the eighth day), it is designated for the whole day.

It is a positive biblical commandment to sit in the Sukkah for seven days, from the 15th of Tishrei until after the 21st of Tishrei, for it is written in Parashat Emor (Leviticus 23:34): "On the fifteenth day of this seventh month is the feast of tabernacles for seven days unto the LORD... Ye shall dwell in booths seven days; all that are home-born in Israel shall dwell in booths; that your generations may know that I made the children of Israel to dwell in booths, when I brought them out of the land of Egypt" (Leviticus 23:42-43).