The Talmud (Bavli Shabbat 21b) explains what is Hanukkah:
What is Hanukkah? For our Rabbis taught: On the twenty-fifth of Kislev commence the days of Hanukkah, which are eight on which a lamentation for the dead and fasting are forbidden.
For when the Greeks entered the Temple, they defiled all the oils therein, and when the Hasmonean dynasty prevailed against and defeated them, they made search and found only one cruse of oil which lay with the seal of the High Priest, but which contained sufficient for one day's lighting only; yet a miracle was wrought therein and they lit the lamp therewith for eight days.
The following year these days were appointed a Festival with the recital of Hallel and thanksgiving.
What else is there to say?