Chanukah Why is it 8 days long?

Here's the classic and timeless question asked by Rav Yosef Karo in his most prodigious work, Bet Yosef (O.H. 670): A flask with one night’s oil burned for 8 nights. But being that there was oil for one night, the miracle actually lasted only 7 nights. So why is Chanukah 8 nights? 

The simplicity of this 500 year old question has brought about much interest and many different answers. I'd like to bring seven of the more famous answers. Here are three answers  from Rav Karo himself:

1. The Cohanim in charge of preparing the Menorah for lighting knew that it would take eight days until new oil could be obtained. They therefore divided the flask into eight parts, so that at least the Menorah would at least be lit every day. A miracle occurred and the small amount of oil that was placed in the Menorah each day lasted an entire day.

2. On the first night, the contents of the jug was emptied into the Menorah. After filling the entire menorah the oil jug was still full of oil.

3. After the first night, when they entered the sanctuary the next morning to clean the menorah, they found that the cups of the menorah were still full of oil, despite having burned the entire night.

4. The Meiri writes that The Greeks defiled and looted the Temple for many days in search of oil to defile. Despite their superiority in strength and numbers, they missed one flask. The Jews entered the Beit HaMikdash and found the one jar of oil that hadn't been defiled straightaway. 

5. The Peri Hadash explains that the seven days commemorate the miracle of the oil, and one day commemorates the miracle that a few weak Jewish soldiers defeated the mighty Greek legions.

6. Chidushei HaRim explains that they knew that the oil wouldn't last eight days so they made the wicks one-eighth of the normal thickness. Nevertheless, the flames burned just as brightly as if the wicks had been the normal thickness.

7. The Arukh HaShulkhan writes that the Mitzvah of Berit Milah was forbidden under the reign of Antiochus, after the military victory, the Jews were once again able to openly perform the commandment. Since the Berit Milah takes place on the 8th day we celebrate Hanukah for 8 days. 

8. In the Book of the Maccabees, it states that the Jews were unable to celebrate Sukkot that year. So the 8 days of Hanukah was established in lieu of the 8 days of Sukkot. (Thanks David Balint)

חנוכה שמח, Happy Chanukah and Hanukah Allegre!