Hanukkah (Hebrew: חנוכה‎, IPA: ['χanuka], alt. Chanukah), also known as the Festival of Lights, is an eight-day Jewish holiday commemorating the rededication of the Holy Temple in Jerusalem at the time of the Maccabean Revolt of the 2nd century BCE. Hanukkah is observed for eight nights, starting on the 25th day of Kislev according to the Hebrew calendar, and may occur from late November to late December on the Gregorian calendar.

The festival is observed by the kindling of the lights of a special candelabrum, the nine-branched Menorah or Hanukiah, one light on each night of the holiday, progressing to eight on the final night. An extra light called a shamash (Hebrew: "guard" or "servant") is also lit each night, and is given a distinct location, usually higher or lower than the others. The purpose of the extra light is to adhere to the prohibition, specified in the Talmud (Tracate Shabbat 21b–23a), against using the Hanukkah lights for anything other than publicizing and meditating on the Hanukkah story. (The shamash is used to light the other lights.) As such, if one were to read from the lights—something prohibited—then it's not clear whether the light one's reading from was from the Hanukkah lights or the shamash light. So the shamash acts as a safeguard from accidental transgression.

From Wikipedia http://en.wikipedia.org/wiki/Hanukkah