The Sages explained why the holiday of Hanukkah was instituted, and why it is celebrated for eight days.

On the twenty-fifth in the month of Kislev is Hanukkah, for eight days, during which one may not eulogize …. When the Greeks entered the Sanctuary they defiled all the oil that was in the Sanctuary. When the house of the Hasmoneans overcame them and emerged victorious over them, they searched and found only one cruse of undefiled oil that was resting there with the seal of the High Priest. It had enough oil to kindle the candelabrum for only one day. But a miracle was performed, and the priests kindled from it for eight days. The next year the Sages instituted them as eight days of festivity….

What did the Sages see that led them to make Hanukkah eight days? In the days of the Greek kingdom, the Hasmoneans entered the Sanctuary and rebuilt the altar and whitewashed it with lime. For all seven days the Hasmoneans dedicated the altar by presenting offerings on it utilizing service vessels.

The dedication by the Hasmoneans is commemorated throughout the generations. Why is it observed throughout the generations? Because they performed the dedication of the altar when they were redeemed from distress to relief, and they then recited psalms of praise, Hallel, and thanksgiving, and they kindled lamps in purity.

Since the Greeks entered the Sanctuary and defiled all the vessels, there was nothing on which to kindle the lamps. When the house of the Hasmoneans overcame them, they brought seven iron skewers, and they plated them with tin, and began to kindle them… (Megillat Ta’anit 9)