Between 21 December and 23 December
The shortest day of the year, where the night is longest, and darkness holds sway over light, is known as the Winter Solstice. It has been observed since before the Roman invasion, acknowledged during the Christianisation of Britain, and holds astrological and meteorological value to this day. As such, it is an important date within both historical and contemporary calendars.
The Solstices at Summer and Winter held the same importance as the Vernal and Autumnal Equinoxes, and it is from Winter Solstice that the date for Christmas was assigned. The Solstices mark the heights of Summer and Winter.
Though Christian nations have thus, from an early period in the history of the church, celebrated Christmas about the period of the winter-solstice or the shortest day, it is well known that many, and, indeed, the greater number of the popular festival observances by which it is characterised, are referrible to a much more ancient origin.Robert Chambers, The Book of Days: A Miscellany of Popular Antiquities
Winter Solstice occurs in December, and marked the height of the strength of the Holly King, who ruled from Autumn to Spring. Both the Holly King and his counterpart, the Oak King, would wax and wane in power, and though the Oak King controlled life through the crops and harvest, the Holly King held sway over death, which was of equal importance and magnitude. At the Winter Solstice cattle and sheep would be slaughtered so they would not require feeding throughout the harshest Winter months, starting in January. This meant more food for people, both in terms of stored crops but also fresh and preserved meat.
The Solstice is still noted today. People wishing to celebrate it often gather at historical sites such as Stonehenge, where the Great Trilithon—the large structure in the centre of the circle—faces the Winter Solstice sunset with its smooth side. There were many ancient rituals related to the Winter Solstice, including traditions for the death and rebirth of sun gods. As such, the Solstice would be marked with a feast, and this was later absorbed into Christmas.