One of the reasons why the celebration of the solstice runs so deep in the human psyche is that people feared, deep down, that the light of the sun might not return. It's also a moment of jollity and colour in the midst of the wet, cold, dark winter.
The winter solstice occurs exactly when the earth's axial tilt is farthest away from the sun at its maximum of 23° 26'. Just about every culture has a festival around the winter solstice.
One of my favourite sites about the winter solstice is Candlegrove, which has reflections on Solstice, Sacaea and Saturnalia, Yule and celebrating the solstice today.
"Shall we liken Christmas to the web in a loom? There are many weavers, who work into the pattern the experience of their lives. When one generation goes, another comes to take up the weft where it has been dropped. The pattern changes as the mind changes, yet never begins quite anew. At first, we are not sure that we discern the pattern, but at last we see that, unknown to the weavers themselves, something has taken shape before our eyes, and that they have made something very beautiful, something which compels our understanding."
--Earl W. Count, 4,000 Years of Christmas