The early Church had no celebration for the birth of Jesus. The date was unknown. However, with the growth of a calendar by which events in the life of Jesus could be commemorated in turn, the birth of Jesus was inevitably given a day, just as the single‐day celebration at Easter was eventually spread through Holy Week. At first, 6 January was observed and a whole sequence of stories was rehearsed—birth, the visit of the Magi, and baptism. But in Rome, where 6 January was set aside for the Magi, it seems that in the 4th cent. CE the Church designated 25 December as the appropriate day for celebrating the Nativity because it had been the day of the pagan festival of Sol Invictus, when the unconquerable sun triumphed annually over the darkness of winter and the days grew longer again. There was a verse in Malachi (4: 2) about the rising of the ‘Sun of Righteousness’ which could then seem to be especially apposite.
It is also relevant that the inauguration of a huge sun‐god statue, the Colossos of Rhodes, took place around sunrise on 25 December in (probably) 283 BCE, and also that a new temple in honour of the sun‐god had been consecrated in Rome on 25 December 274 CE. Under the emperor Constantine (d. 337 CE) Christ was venerated as the Sun of Righteousness, and there is a mosaic in the crypt of St Peter's in the Vatican where Christ is portrayed adorned with sun‐rays and riding in a chariot.