Hanukkah is about God's protection of the Israelites, and the miracles that occurred on the day.
The holiday celebrates the triumph of faith and courage over military might, when a band of Israelites stood up for their right to be Jewish. They were prohibited under penalty of death from studying their sacred texts or performing important mitzvot. Their holy Temple had been defiled, and they were ordered to worship other gods. However, a small band of faithful Israelites, known as the Maccabees, rose up and defeated the invaders, reclaimed the Temple, and rededicated it to God. The eternal flame in the Temple's great menorah (lamp stand) had to be lit. But the sacred olive oil needed to burn in the lamp stand took 8 days to press and purify. The Jews had only a one-day supply of oil. They decided, in faith, to light the flame anyway. And, a great miracle occurred. The jug of oil refilled itself every day with enough oil to relight the Temple's great menorah, and this continued for 7 days, the exact time it took to prepare new oil! It is a common misconception that the oil burned continuously for 8 days.
This story is even mentioned by Josephus, the first century Jewish historian.Since that time, Hanukkah has been celebrated for 8 days to recall the miracle when the menorah burned for 8 days at the Temple. The main miracle of Hanukkah is the victory of the Maccabees against the most powerful army in the world.