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?
Looking at your world from Above, all is good.
Looking at your world from within, things don’t always look so nice.
Until you connect your world below to the world above. Then the goodness flows downward without distortion.
How do you make that connection? By clinging tightly above.
By putting all your trust in G‑d.