The date: On the twenty fifth of Kislev begins the eight days of Chanukah.
What it commemorates: Chanukah commemorates the miracles that occurred during the reign of the Syrian Empire in times of the second Temple. They made decrees against Jews, preventing them from following Torah and Mitzvos. Their money and daughters were used at the Syrians’s discretion. They entered into the Heichal of the Temple and defiled its contents. Hashem then had mercy on His people and saved us from their hands. He gave strength to the small army of the Chashmonaim Kohen family to battle and overcome the great Syrian army. The day of the Syrian army’s final defeat was on the 25th of Kislev. When the Jews entered into the Temple they found only one flask of pure oil. This oil was enough to last one day. A miracle occurred and the oil lasted for eight day which was the amount of time it took them to retrieve more pure oil. For this reason the Sages in that generation decreed upon the Jews to celebrate eight days of Chanukah beginning from the 25th of Kislev, each year.
The meaning behind its name: The name Chanukah is a Hebrew abbreviation for “they rested from their enemies on the 25th of Kislev”. An additional meaning of Chanukah is dedication, implying that at that time the temple and alter were rededicated after their defilement by the Greeks.
Why is Chanukah celebrated as 8 days if there was enough oil to naturally last for one day?
There are a number of answers offered to this question:
- Some say that they split the oil to 8 parts for 8 days and nevertheless each day it lasted a full day.
- Others say that they used all of the oil the first day, but nevertheless miraculously some of the oil was left unconsumed in order for the following day’s miracle of lighting to be naturally bound.
- Others explain they used all the oil the first day but only 1/8th of the oil became consumed each day.
- Some say that miraculously the jar refilled with oil after having emptying it.
- Others say that oil of the candles simply never became consumed by the fire. [The Rebbe explains this to mean that the oil was consumed but remained at the same time, similar to the miracle in the Temple that the Aron did not take up space in the Kodesh Hakadashim.]
- The fact that they actually found a pure jar of oil is itself a miracle, and that is the miracle of the first day.
- They compared the holiday to Sukkos which is 8 days.
- The celebration on the 1st day is for the victory against the Greeks from whom they rested from battle on that day.
 M”B 670/1 taken from Rambam
 When was the war won and consequently the Menorah lit? According to most Rishonim [Meiri Shabbos 21b] the war was won by the Chashmonaim on the 24th of Kisleiv and the Menorah was lit on the night of the 25th and then again the next day on the day of the 25th. However according to the Rambam [Hilchos Chanukah 3/2] the war was only won in the midst of the 25th and thus the first lighting was done on the afternoon of the 25th which was the eve of the 26th. [See Peri Chadash 670; M”B 670/1 writes the war was won on the 25th; Rebbe in Shaareiy Moadim Chanukah p. 39 ]
 M”B 670/1
 First answer in Beis Yosef.
 Taz 670/1 brought in Hashlamas Rav Nechemiah 670
 As blessing can only befall onto an already existing item. Hence there must have been some oil miraculously left over from the previous night. [ibid]
 Peri Chadash 670/1
 Second answer in Beis Yosef.
 Third answer of Beis Yosef
 Likutei Sichos 15 p. 183 [Shaareiy Moadim p. 128]
 Rokeich brought in Bnei Yisaschar
 Chayeh Adam brought in Taamei Haminhagim 843; Peri Chadash 670/1