This is the period lasting from the evening of December 24th, Christmas Eve, through until January 5th, the eve of the Epiphany.
In Egypt in 1996 BC the calendar recorded the winter solstice as being on 6 January, but by the time Alexandria was founded in 331 BC the inaccuracy of the calendar meant that the solstice was on 25 December. The dates of the Christian festivals of Christmas and Epiphany are both linked with the winter solstice, transmuted celebrations of the pagan world. It is at Rome in the early fourth century that we find firm evidence of the celebration of Christmas. In the year 274, the emperor Aurelian introduced in the imperial capital the festival of the Invincible Sun, Natalis Solis Invicti, on 25 December. At some point before 336 the church must have established on this date the commemoration of the incarnation, the birth of the Son of Righteousness.
The evidence for this is contained in a martyrology written by Philocalus in 354 giving two lists of anniversaries, the one detailing the burial days of Roman bishops and the other those of martyrs. He based this work on and added to a list whose latest date was 335. Both lists are set out in the order of the months, and it is highly significant that the opening date in the first is 27 December, and in the second 25 December, where the entry makes reference to the nativity. From this it is possible to conclude that by 336 Christmas was in existence in Rome, and was the beginning of the liturgical year. Some scholars have argued that the identification of 25 December as the date of the birth of Christ predates this winter festival of the sun, and originates in circumstances unlikely to be influenced by it. It can be noted that both Hippolytus and Tertullian identify the date of the crucifixion as 25 March; if one then adopts that as the date of the conception of Jesus as well, 25 December becomes the date of his birth. Be that as it may, there seems no doubt that the Christianization of the pagan festival of the sun gave the celebration of Christmas on that date an enormous boost even if it was not the original reason for it; the existence of an earlier tradition of celebrating the birth of Christ on this date would have made the transition even easier.
By tradition, the Christmas season is regarded as consisting of twelve days, and ends with the Epiphany. The liturgies of this period are woven around the infancy narratives from the gospels of Matthew and Luke, together with all those passages that were seen to anticipate the incarnation in the Old Testament and expound its significance in the New. It remains the most widely observed of all Christian feasts, perhaps because this festival combines human need for a party to enliven the gloom of midwinter, the natural human response to childbirth, and the message of ‘Immanuel’ – ‘God with us’. It was a stroke of sheer pastoral genius to Christianize the pagan midwinter festival in this way. The midnight Eucharist of Christmas is probably the most commonly attended of all liturgical observances.