Winter solstice

Winter solstice was the time when the hours of daylight were the shortest, and would soon increase. On the Northern Hemisphere of Earth, it was celebrated December 21st. (TV: A Girl's Best Friend) It was known to have been celebrated on a planet controlled by the Sardick family in the 44th century. The Eleventh Doctor and Kazran Sardick both called this a celebration of being, "Halfway out of the dark". (TV: A Christmas Carol)