Why Is Chanukah Eight Days Long?

This question is as old as the holiday itself . . . If the pure oil which was found was enough for one day, then seemingly no miracle occurred on the first day of Chanukah. Why, then, isn’t Chanukah celebrated for seven days—starting with the 26th of Kislev, when the miracle began?

Many answers are given for this glaring difficulty. In fact, since Rabbi Yosef Caro (16th-century author of the Code of Jewish Law) posed this question (and suggested three answers of his own), over 100 other answers have been proposed!

The following are a few of them:1

1. Considering that it would take another eight days to procure pure oil, the Jews divided the jug of oil—which contained enough oil for only one night—into eight equal amounts. They figured that they would light one-eighth of the oil each of the next eight nights. Miraculously, on each of the eight nights of Chanukah, the oil which should have lasted only one-eighth of the night lasted for the entire night.
2. After they filled the menorah with oil on the first night, the jug remained full. The same happened on the ensuing seven days.
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. We celebrate the first day to commemorate the miraculous victory over the Greeks.

For an in-depth (and entertaining) exposition of these solutions, see The Menorah Files.