Answer:
Channukah is celebrated for eight days. The popular story as to why it is celebrated for eight days because of the miracle that occurred with the oil for the menorah that stood in the Temple.
The menorah was meant to be lit 24 hours a day, 7 days a week. However, once the Temple was reclaimed from the Assyrian invaders and rededicated, there was only enough oil to last for one day, and it would take eight to make more. The oil that they had ended up burning for eight days in the Temple's menorah, the perfect amount of time until the new oil could be made.
However, the true reason that Channukah lasts for 8 days is related to the holiday of Sukkot. Channukah lasts 8 days because during the Assyrian occupation, the holiday of Sukkot could not be celebrated. As a result, once the Temple was reclaimed from the invaders and rededicated, there was a postponed observance of that holiday which is 8 days.