Winter Solstice, usually December 21 or 22, is the longest night of the year. It marks the beginning of winter; but it also marks the turning of the year, when days start getting longer again and the earth heads back toward spring. In older times, when lives were more dependent on the cycles of nature and agriculture, this was celebrated as the rebirth of the mighty Sun, the Lord of Light.
The Christian church absorbed many pagan traditions in the effort to win folk over to the new faith. Many of the aspects of the ancient mid-winter Yule celebrations became Christmas traditions, part of celebrating "Christ Mass," the birth of the mighty Son.
Today, there is a renewal of interest in the ancient traditions among people who feel that modern life has become unhealthily separated from the cycles of nature, and who celebrate the ancient traditions of Yule on Winter Solstice.