The festival of Sukkot, commemorating G-d's enveloping protection of the Children of Israel during their 40-year journey through the desert (1313-1273 BCE), is celebrated for seven days, beginning from the eve of Tishrei 15. During this time, we are commanded to "dwell" in a sukkah -- a hut of temporary construction, with a roof covering of raw, unfinished vegetable matter (branches, reeds, bamboo, etc.) -- signifying the temporality and fragily of human habitation and man-made shelter and our utter dependence upon G-d's protection and providence. "How [does one fulfill] the mitzvah of dwelling in the sukkah? One should eat, drink, and live in the sukkah, both day and night, as one lives in one's house on the other days of the year: for seven days a person should make his home his temporary dwelling, and his sukkah his permanent dwelling" (Code of Jewish Law, Orach Chaim 639:1).
At least one k'zayit (approx. 1 oz.) of bread should be eaten in the sukkah on the first evening of the festival, between nightfall and midnight. A special blessing, Leishiv BaSukkah, is recited. For the rest of the festival, all meals must be eaten in the sukkah (see the Code of Jewish Law or consult a Halachic authority as to what constitutes a "meal"). Chabad custom is to refrain from eating or drinking anything outside of the sukkah, even a glass of water.
Also see: the Ushpizin