Commentary on al-Fāsī: On the Babylonian Talmud, Tractate Shabbat

The next year they established them [i.e., the days of Hanukkah] as eight festive days. [b. Shabbat 21b]

All eight days were included, as no such miracle had ever been wrought for Israel, for even though Pharaoh did not decree against the observance of the commandments, the evil Greeks did so decree. Therefore, they extended these days to match the length of the festival of Sukkot, which is eight days, to remember the miracle on them [i.e., each of the eight days].

Even if the miracle of the oil had not occurred for them, they would still have rejoiced and commemorated the miraculous events for a period equivalent to the longest of all the festivals. However, they would not have lit lights at all; rather, they would have praised and exalted God as on the other festivals. When they received the further miracle of the oil, they added that one should light lights, to commemorate the double miracle.

The Jerusalem Talmud explains that, as the people were located four days’ distance from the place where the oil was prepared, they were unable to bring enough good oil to Jerusalem within eight days. During these eight days that the miracle transpired, they sent messengers there and brought a large quantity of oil for them from olives that had been picked after the overbearing Greeks were defeated.