On the 25th of Kislev in the year 3622 from creation, the Maccabees liberated the Holy Temple in Jerusalem, after defeating the vastly more numerous and powerful armies of the Syrian-Greek king Antiochus IV, who had tried to forcefully uproot the beliefs and practices of Judaism from the people of Israel. The victorious Jews repaired, cleansed and rededicated the Temple to the service of G-d. But all the Temple's oil had been defiled by the pagan invaders; when the Jews sought to light the Temple's menorah (candelabra), they found only one small cruse of ritually pure olive oil. Miraculously, the one-day supply burned for eight days, until new, pure oil could be obtained. In commemoration, the Sages instituted the 8-day festival of Chanukah, on which lights are kindled nightly to recall and publicize the miracle.
Link: The Story of Chanukah
Today is the first of the two Rosh Chodesh ("Head of the Month") days for the month of Tevet (when a month has 30 days, both the last day of the month and the first day of the following month serve as the following month's Rosh Chodesh).
The Yaaleh V'yavo prayer is added to the Amidah and to Grace After Meals, and the additional Musaf prayer is said (when Rosh Chodesh is Shabbat, special additions are made to the Shabbat Musaf). Because it is also Chanukah today, the "full" Hallel (Psalms 113-118) is recited (and not the "partial Hallel" said on the Rosh Chodesh days of other months).
Many have the custom to mark Rosh Chodesh with a festive meal and reduced work activity. The latter custom is prevalent amongst women, who have a special affinity with Rosh Chodesh -- the month being the feminine aspect of the Jewish Calendar.
Links: The 29th Day; The Lunar Files
This Shabbat is unique in that three Torah scrolls are taken from the ark and read from in the public Torah reading: one scroll for the weekly Parshah, a second scroll for the Rosh Chodesh reading, and a third scroll for the Chanukah reading. (The only other occasions on which three scrolls are taken out are Simchat Torah, and when Rosh Chodesh Adar or Rosh Chodesh Nissan fall on Shabbat).
Special prayers of thanksgiving -- Hallel (in its full version) and V'Al HaNissim -- are added to the daily prayers and Grace After Meals on all eight days of Chanukah. Tachnun (confession of sins) and similar prayers are omitted for the duration of trhe festival.
In commemoration of the miracle of Chanukah (see "Today in Jewish History" for Kislev 25) we kindle the Chanukah lights -- oil lamps or candles -- each evening of the eight-day festival, increasing the number of lights each evening. Tonight we kindle seven lights. (In the Jewish calendar, the day begins at nightfall; this evening, then, commences the 7th day of Chanukah).
IMPORTANT: Because of the prohibition to kindle fire on Shabbat, the Chanukah lights must be lit after the Havdalah service marking the end of Shabbat at nightfall.
Links:
Where the Infinite Light is hidden
—there you see a world.
If you could see the light that generates this world
You would not see any world at all.
And yet the world you see
is nothing more than G-d's own hand hiding His light.
That is all there is to your world:
G-d hiding behind Himself.
Why?
So that you will search for Him
and find Him in this world of yours.