The word Ḥanuka means 'dedication' and it is used in this sense in Hebrew in phrases like 'Ḥanukat-haBayit', dedication of one's new home.
So, what dedication are we referring to in our holiday "Ḥanuka"?
During the Second century BCE the Jews in Israel lived under the rule of the Syrian-Greek army of Antiochus Epiphanes. They were not permitted to practice their religion and at one point the Bet-haMiqdash (The Holy Temple of Jerusalem) was captured and defiled by the Greeks. The Greeks introduced an image of their pagan god, Zeus, and dedicated our Holy Temple to him, offering sacrifices of impure animals like pigs.
In the years 165 BCE the Jews lead by Yehuda Makabi rebelled against the powerful armies of Antiochus and with God's help defeated them. When they regained possession of the Bet haMiqdash, they purified the Holy Temple. In order to re-dedicate the Temple to God Almighty they needed to light the Menora, which indicates that the Bet haMiqdash was fully operating to God's service. They found a small jar, with an amount of pure oil which would normally last for one night only.
They lit the Menora and joyfully dedicated the Bet haMiqdash back to God. The Makabim thought that they will need to interrupt the dedication of the Temple until new oil could be produced. Miraculously, the oil lasted for eight days, the exact time needed to make new pure olive oil.
Ḥanuka then celebrates the 'dedication (or re-dedication) of the Bet haMiqdash' to HaShem after years of being defiled.
Ḥanuka is observed by the kindling of candles during the eight nights of the holiday, in remembrance of the miracle of the oil.
Ḥanuka is celebrated on the 25th of the month of Kislev.
This year, 2012, Ḥanuka begins Saturday, December 8, once Shabbat is over.
Watch this beautiful video THIS IS YOUR LIGHT from Aish.com