The winter solstice is the reversal of the Sun's apparent ebbing in the sky, in ancient times it was seen as the symbolic death and rebirth of the Sun or of a Sun God In cultures which used cyclic calendars based on the winter solstice, the "year as reborn" was celebrated with reference to life-death-rebirth deities or "new beginnings" .
Some important Neolithic and early Bronze Age archaeological sites in Europe are associated with the winter solstice, such as Stonehence in England and Newgrange in Ireland. The primary axes of both of these monuments seem to have been carefully aligned on a sight-line pointing to the winter solstice sunrise (Newgrange) and the winter solstice sunset (Stonehenge). It is significant that at Stonehenge the Great Trillithon was oriented outwards from the middle of the monument, i.e. its smooth flat face was turned towards the midwinter Sun.
In the ancient Roman calendar, December 25 was the date of the winter solstice.[8][9] In AD 274, the emperor Auralian made this the date of the festival Dies Natalis Solis Invicti, the birthday of Sol Invictus or the 'Invincible Sun'. Gary Forsythe, Professor of Ancient History, says "This celebration would have formed a welcome addition to the seven-day period of the Saturnalia (December 17–23), Rome's most joyous holiday season since Republican times, characterized by parties, banquets, and exchanges of gifts".[9]
A widely-held theory is that the Church chose December 25 as Christ's birthday (Dies Natalis Christi) to appropriate the Roman winter solstice festival marking the sun god's birthday (Dies Natalis Solis Invicti). According to C. Philipp E. Nothaft, a professor at Trinity College Dublin, this theory "is nowadays used as the default explanation for the choice of 25 December as Christ's birthday.