The date:[1]

On the twenty fifth day of Kislev begins the festival of the eight days of Chanukah.

What it commemorates:[2]  Chanukah commemorates the miracles that occurred during the reign of the Greek-Syrian Empire [The Seleucid 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 Greeks’ discretion. They entered 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 Greek-Syrian army. The day of the Syrian army’s final defeat was on the 25^th of Kislev.[3] When the Jews entered the Temple they discovered a single flask of pure oil. This oil was enough to last for one day. A miracle occurred, and the oil lasted for eight days. For this reason, the Sages in that generation decreed upon the Jews to annually celebrate eight days of Chanukah beginning from the 25^th of Kislev. Alternatively, some Poskim[4] explain that the festival was established [not due to the miracle of the candles but rather] in celebration of the completion of the Tabernacle in the times of Moses, and the reinaugurated of the altar by the Chashmonaim after its defilement by the Greeks, which took place on the 25^th of Kisleiv.

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[1] 670:1

[2] M”B 670:1 taken from Rambam

[3] When was the war won and consequently the Menorah lit? Majority of Poskim explain that the war was won by the Chashmonaim on the 24^th of Kisleiv, and the Menorah was lit on the night of the 25^th and then again, the next day, on the day of the 25^th. [Most Rishonim; Meiri Shabbos 21b; Sefer Hamitzvos Tzemach Tzedek Mitzvah 58; Implication of Admur in Likkutei Torah Tzav 16a who says we celebrate on the day of rest and not the day of the victory; See Likkutei Sichos 18 p. 142] However, some Poskim explain that the war was only won in the midst of the 25^th and thus the first lighting was done on the afternoon of the 25^th which was the eve of the 26^th. [Rambam Hilchos Chanukah 3:2; See Peri Chadash 670; M”B 670:1 writes the war was won on the 25^th; See Rebbe in Shaar HaMoadim Chanukah p. 39; Likkutei Sichos 10 p. 142]

[4] Rashi Megillah 30b; Shibulei Haleket 174; Mordechai; Or Zarua 2:321; Darkei Moshe 670; Maharsha Shabbos 21b; Megillas Taanis 9; Elya Raba 670:17; See Kaf Hachaim 670:12; See Likkutei Sichos 20:632 [printed in Shulchan Menachem 3:260]