Nearly every December observance is a festival of light against the year's longest darkness. Christmas on the twenty fifth anchors the month for the Christian world, with Advent leading in, the twelve days of Christmastide flowing out, and Las Posadas, the Mexican novena from the sixteenth to the twenty fourth, walking Mary and Joseph's search for shelter from house to house each evening. Three days before Christmas week begins, Our Lady of Guadalupe on the twelfth of December commemorates the apparitions of 1531 on the hill of Tepeyac and the image on Juan Diego's tilma, the devotion at the heart of Mexican Catholicism and, by papal declaration, of the Americas as a whole.
Hanukkah, the eight night festival of lights, remembers the rededication of the Temple in Jerusalem in 164 BCE after the Maccabean revolt, one lamp added each night; it begins on the twenty fifth of Kislev and falls wholly or partly in December in most years. The winter solstice itself, on or near the twenty first, is Yule, the old Germanic midwinter whose name survives in the Yule log and in the Scandinavian word for Christmas, and modern Pagans keep the solstice as the Wheel of the Year's station of the sun's rebirth, the longest night turning back toward light.
The month, and the year, close with thresholds: Watch Night services on the thirty first carry congregations across midnight in prayer, and the secular new year inherits the shape of every vigil that came before it.
Sources The Oxford Dictionary of the Christian Church, 3rd revised editionF. L. Cross and E. A. Livingstone (eds.) with Our Lady of Guadalupe: The Origins and Sources of a Mexican National Symbol, 1531-1797Stafford Poole, Encyclopaedia Judaica, 2nd editionMichael Berenbaum and Fred Skolnik (eds.)