Sukkot lasts for seven days. The two days following the festival are separate holidays, Shemini Atzeret and Simkhat Torah, but are commonly thought of as part of Sukkot. The holiday commemorates the forty-year period during which the children of Israel were wandering in the desert, living in temporary shelters. Sukkot is also a harvest festival, and is sometimes referred to as Chag Ha-Asif, the Festival of Ingathering. No work is permitted on the first and second days of the holiday.
Work is permitted on the remaining days.
These intermediate days on which work is permitted are referred to as Chol Ha-Mo'ed, as are the intermediate days of Passover. In honor of the holiday's historical significance, we are commanded to dwell in temporary shelters, as our ancestors did in the wilderness. A sukkah must have at least three walls covered with a material that will not blow away in the wind. Canvas covering tied or nailed down is acceptable and quite common in the United States. A sukkah may be any size, so long as it is large enough for you to fulfill the commandment of dwelling in it.