Today is the first day of the festival described below.
The name Succot is the plural of succah which is a temporary hut or booth. The observance of the festival is best described in the passage:

On the fifteenth day of the seventh month, when you have gathered in the fruit of the land, you shall keep a feast to the Lord seven days; on the first day shall be a solemn rest, and on the eighth day shall be a solemn rest. And you shall take on the first day the fruit of goodly trees, branches of palm trees, and the boughs of thick trees, and willows of the brook; and you shall rejoice before the Lord your God seven days. And you shall keep it a feast to the Lord seven days in the year. It shall be a statute forever in your generations; you shall celebrate it in the seventh month. You shall dwell in booths (Succot) seven days; all who are Israelites born shall dwell in booths, that future generations may know that I made the people of Israel to dwell in booths when I brought them out of the land of Egypt.