The great DovBear brings the real reason for the very first Chanucah - which has nothing to do with a miracle of oil:
Chanuka is an 8-day celebration because the Hashmonaim [Hasmoneans] modeled their original holiday on Sukkos. This... is attested to in the Book of Maccabees II 1:8 where we find a quote from a letter sent by the Hashmonaim to other Jews in which they introduce a new holiday called "Sukot b'Kislev":
In Maccabees II 10:6-9 an explanation for this designation is provided. After retaking Jerusalem and the temple...:
...they kept eight days with joy, after the manner of the feast of the tabernacles, remembering that not long before they had kept the feast of the tabernacles when they were in the mountains, and in dens like wild beasts. Therefore they now, carried boughs, and green branches, and palms [i.e. lulavim] for Him that had given them good success in cleansing his place. And they ordained by a common statute, and decree, that all the nation of the Jews should keep those days every year
In other words, they missed out the pilgrimage to Jerusalem and to the Temple on Sukkot, because the revolt was in full swing and the Temple had been desecrated - and so decided to recreate the festival in the month of Kislev, during the winter.
The story of the miracle of oil comes from the Talmud, far later, and is not mentioned in the Book of the Maccabees.