During the time of the second Bet Hamiqdash, the Greek empire forced many decrees upon the nation of Yisrael. The evil Greek empire oppressed the Jewish people both spiritually and physically. Spiritually, they did not permit the Jewish people to worship Hashem through His misvot, or learn Torah. Physically, they looted Jewish homes and businesses. Hanukah stresses the spiritual desecration, for this damage was much greater than any physical damage they caused.
The Greeks entered the Bet Hamiqdash and defiled the pure oil. The Hashmonaim, a group of zealot Kohanim, led an insurgence against the Greeks. They won a battle despite insurmountable odds. When they reclaimed the Bet Hamiqdash, they found only one small canister of pure oil to light the Menorah of Bet Hamiqdash.
Oil for one day lasted for eight days, giving the Kohanim the time they needed to retrieve olives and process them into new pure oil. These eight days are commemorated and known as Hanukah. Our rabbis have declared these days to be days of joy and happiness, when fasting an eulogizing are not allowed.
Enjoy learning more about Hanukah: