As per the Talmud, the month of Kislev marks the onset of the winter season in the Holy Land and is the third month of the "Season of the Rains."
Link: Winter
For the first time since suffering a major heart attack five weeks earlier, on the eve of Shemini Atzeret, the Rebbe left his office in 770 Eastern Parkway and returned to his home, signaling his recovery. Chassidim all over rejoiced at the good news.
From that day on, the Rebbe redoubled his efforts on behalf of the Jewish nation and all of humanity, and for the dissemination of Torah and chassidism. From then on, the first of Kislev is celebrated as a day of thanksgiving and rejoicing.
Link: Illness and Challenge (from the timeline "biography of ideas" in Therebbe.org).
Today is the second of the two Rosh Chodesh ("Head of the Month") days for the month of Kislev (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).
Special portions are added to the daily prayers: Hallel (Psalms 113-118) is recited -- in its "partial" form -- following the Shacharit morning prayer, and the Yaaleh V'yavo prayer is added to the Amidah and to Grace After Meals; the additional Musaf prayer is said (when Rosh Chodesh is Shabbat, special additions are made to the Shabbat Musaf). Tachnun (confession of sins) and similar prayers are omitted.
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 Jewish Month of Kislev
The Laws of Rosh Chodesh
How Does a Jewish Woman Celebrate Rosh Chodesh?
Naturally, we think of the Jewish people as a conglomerate of many Jews. But the Baal Shem Tov saw the Jewish people as a single, indivisible whole.
Think of a geometrical point. A point is indivisible, but not because it is too hard, too big, or too small to cut up. A point simply has no area to be divided. That’s what makes it a point.
And yet, from a point you can extend infinite lines radiating in infinite dimensions.
In a somewhat similar way, but far beyond, all Jews are one Jew. Which means that in any one Jew, you will find all of us—just from a different angle.
So that whatever happens to any one of us instantaneously happens to the entire Jewish people. Not by some ripple effect or resonance. But because any one sample of the whole is the whole and the whole is one.
And so, the Baal Shem Tov taught, when the light of any one Jewish soul breaks free, the entire nation is redeemed along with it.
And accordingly, the Rebbe wrote, the ultimate exodus of our entire people is also a personal, intimate liberation for every Jew.