Winter solstice, arguably the oldest of all the festivities in December. What was it that made people build observatories of wood and stone, more than 4000 years ago, to observe winter solstice, in its most ancient meaning? In the Northern areas, where the fluctuation of the day's length was not only apparent, but dominating in almost any aspect of life, winter solstice was sometimes connected to the re-birth of the sun god. Even in the ancient Greek mythology, although farer in the South, winter solstice was an important date, when all gods gathered on Mount Olympus.
Wikipedia lists 39 festivals worldwide (some of which ancient by themselves today), which are either direct incarnations of winter solstice, or lend important parts of their celebration from it (Christmas and Hanukkah among them); and there were certainly many more in prehistoric times.
Apart from the symbolical meaning of hope and rebirth; and the fact that it connects so many religious holidays with each other; we should also recall the scientific cause, why winter solstice is observed in so many cultures about the same time: It stems from the inclination of Earth. And thus it should remind us of the fact that we are all inhabiting the same small, rotating marble.