Why is Sukkot celebrated for 8 days?

Inside of Israel it is only seven days as stated in the Torah in Vayikra (Leviticus) Chapter 23 (text copied from Chabad.org http://www.chabad.org/library/bible_cdo/aid/9924) 42.For a seven day period you shall live in booths. read more

Thus, to ensure the festival was observed on the correct day, they kept an extra day of each festival to make sure that they held it on the correct day. This is why outside of Israel we have two days of each Yom Tov- and not just one ensure it was held on the correct day. read more

Sukkot (Hebrew: סוכות ‎ or סֻכּוֹת, sukkōt, commonly translated as Feast of Tabernacles or Feast of the Ingathering, traditional Ashkenazi pronunciation Sukkos or Succos, literally Feast of Booths) is a biblical Jewish holiday celebrated on the 15th day of the seventh month, Tishrei (varies from late September to late October). read more

Sukkot lasts for seven days. The two days following the festival, Shemini Atzeret and Simchat Torah, are separate holidays but are related to Sukkot and are commonly thought of as part of Sukkot. The festival of Sukkot is instituted in Leviticus 23: 33 et seq. read more