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?
Know as clear as day and contemplate this deeply, that He who emanates all into being is He who rules and judges this world. In the heavens above and on the earth below, there is nothing else. (Deut. 4:39)
The same G‑d who transcends all time and space is the same G‑d manifest in all the forces and fields of physics.
The same G‑d from whom all existence emerges is the same G‑d who conceals that truth from every conscious being, providing space for each one to be that which it is.
The same G‑d from whom all life extends is the same G‑d who restrains that energy, with kindness, so that finite creatures can exist, to the tiniest worm hiding in the bowels of the earth.
And when you know all this, and you contemplate it well…
…that the very substance of each thing is nothing more than an articulation of divine energy, that even time and space extend from Him at will, as do the very limitations and sense of existence of each creation, that darkness is His presence as much as light, hiddenness as is revelation, matter as is energy, somethingness as is nothingness, for He is both beyond and within each place, moment and thing…
…then you will know that there really is nothing else. Nothing else but the one G‑d.
As the Baal Shem Tov would say in simple, succinct form: Divine energy is all there is, and everything is divine energy.