What is Hanukkah?
On the 25th of Kislev are the days of Hanukkah, which are eight... these were appointed a Festival with Hallel [prayers of praise] and thanksgiving. —Shabbat 21b, Babylonian Talmud
Hanukkah commemorates the rededication of the Temple of Jerusalem in 165 BCE after the Temple had been profaned by the Hellenistic Emperor Antiochus IV. At the time of the rededication, there was just one day's worth of pure oil left that was needed to keep the Temple menorah burning. But tradition says a great miracle happened: It lasted for eight days.
Each year, Jews light candles in their hanukkiot (Hanukkah menorahs) to symbolize the miracle. On the first night of Hanukkah, one candle is placed in the menorah. On each successive night, another candle is added. By the last night of Hanukkah, eight candles are glowing brightly in celebration of this beautiful festival.