According to Jewish tradition, Hanukkah began as a celebration of the liberation of the Second Temple in Jerusalem. The story goes that the altar needed oil to be cleansed but oil for only one day was found. This oil burned for eight days, starting the eight-day celebration of Hanukkah.
Hanukkah commemorates the victory of the Maccabees over the Greek and Syrian forces of King Antiochus in 165 B.C.E. Antiochus put an idol of Zeus in the Jewish temple, forbade circumcision and forced Jews to eat food that was not kosher. Many Jews adapted to this culture, but many still opposed it.
The opposing group was called the Maccabees and were led by a man named Mattathias and his brothers. On the 25th day of the Jewish month Kislev, which falls on December in a modern calendar, the Maccabees reclaimed and rededicated the temple.
Some modern theories argue that Antiochus was intervening in a civil war between the Maccabean Jews and the Hellenized Jews. According to these scholars, Antiochus sided with the Hellenized Jews against the more traditional Maccabean Jews to take control of their temples.
Many consider the miracle of the oil a legend, and it's truthfulness has been in question since the Middle Ages. Regardless, the miracle is still retold every year during the Hanukkah celebration.