What is Chanukah?

Beginning with the 25th of Kislev, eight days of Chanukah are observed, during which no eulogies are delivered, nor is fasting permitted. For when the Greeks entered the Sanctuary, they defiled all the holy oils used for the Menorah in the temple, and when the Hasmonean house prevailed and vanquished them, they searched and found only one remaining jar of oil with the Kohen Gadol's seal.

Although it contained only enough oil to burn for one day, a miracle occurred, and the oil burned for eight days. A year later they (the Rabbis) designated these days as Yamim Tovim (holidays) on which praise and thanksgiving were to be said." (Tractate Shabbat 21)