One theory to explain the choice of 25 December for the celebration of the birth of Jesus is that the purpose was to Christianize the pagan festival in Rome of the Dies Natalis Solis Invicti, meaning "the birthday of the Unconquered Sun", a festival inaugurated by the Roman emperor Aurelian to celebrate the sun god and celebrated at the winter solstice, 25 December. According to this theory, during the reign of the emperor Constantine, Christian writers assimilated this feast as the birthday of Jesus, associating him with the "sun of righteousness" mentioned in Malachi 4:2.
An explicit expression of this theory appears in an annotation of uncertain date added to a manuscript of a work by 12th-century Syrian bishop Jacob Bar-Salibi. The scribe who added it wrote: "It was a custom of the Pagans to celebrate on the same 25 December the birthday of the Sun, at which they kindled lights in token of festivity. In these solemnities and revelries the Christians also took part. Accordingly when the doctors of the Church perceived that the Christians had a leaning to this festival, they took counsel and resolved that the true Nativity should be solemnised on that day." This idea became popular especially in the 18th and 19th centuries.
In the judgement of the Church of England Liturgical Commission, this view has been seriously challenged by a view based on an old tradition, according to which the date of Christmas was fixed at nine months after 25 March, the date of the vernal equinox, on which the Annunciation was celebrated. This alternative view is considered academically to be "a thoroughly viable hypothesis", though not certain. The Jewish calendar date of 14 Nisan was believed to be that of creation, as well as of the Exodus and so of Passover, and Christians held that the new creation, both the death of Jesus and the beginning of his human life, occurred on the same date, which some put at 25 March in the Julian calendar. It was a traditional Jewish belief that great men lived a whole number of years, without fractions, so that Jesus was considered to have been conceived on 25 March, as he died on 25 March, which was calculated to have coincided with 14 Nisan. Sextus Julius Africanus (c. 160 – c. 240) gave 25 March as the day of creation and of the conception of Jesus.[86] In his work Adversus Haereses, Irenaeus (c. 130–202) identified the conception of Jesus as March 25 and linked it to the crucifixion at the time of the equinox, with the birth of Jesus nine months after on 25 December at the time of the solstice. An anonymous work known as De Pascha Computuslinked the idea that creation began at the spring equinox, on 25 March, with the conception or birth of Jesus on 28 March, the day of the creation of the sun in the Genesis account. One translation reads: "O the splendid and divine providence of the Lord, that on that day, the very day, on which the sun was made, the 28 March, a Wednesday, Christ should be born. For this reason Malachi the prophet, speaking about him to the people, fittingly said, 'Unto you shall the sun of righteousness arise, and healing is in his wings.'"[8][88] The tractate De solstitia et aequinoctia conceptionis et nativitatis Domini nostri Iesu Christi et Iohannis Baptistae falsely attributed to John Chrysostom also argued that Jesus was conceived and crucified on the same day of the year and calculated this as 25 March. This anonymous tract also states: "But Our Lord, too, is born in the month of December ... the eight before the calends of January [25 December] ..., But they call it the 'Birthday of the Unconquered'. Who indeed is so unconquered as Our Lord...? Or, if they say that it is the birthday of the Sun, He is the Sun of Justice." A passage of the Commentary on the prophet Daniel by Hippolytus of Rome, written in about 204, has also been appealed to.
With regard to a December religious feast of the sun as a god (Sol), as distinct from a solstice feast of the (re)birth of the astronomical sun, one scholar has commented that, "while the winter solstice on or around December 25 was well established in the Roman imperial calendar, there is no evidence that a religious celebration of Sol on that day antedated the celebration of Christmas".[91] "Thomas Talley has shown that, although the Emperor Aurelian's dedication of a temple to the sun god in the Campus Martius (C.E. 274) probably took place on the 'Birthday of the Invincible Sun' on December 25, the cult of the sun in pagan Rome ironically did not celebrate the winter solstice nor any of the other quarter-tense days, as one might expect." The Oxford Companion to Christian Thought remarks on the uncertainty about the order of precedence between the religious celebrations of the Birthday of the Unconquered Sun and of the birthday of Jesus, stating that the hypothesis that 25 December was chosen for celebrating the birth of Jesus on the basis of the belief that his conception occurred on 25 March "potentially establishes 25 December as a Christian festival before Aurelian's decree, which, when promulgated, might have provided for the Christian feast both opportunity and challenge".
The Chronography of 354, an illuminated manuscript compiled in Rome, is an early reference to the date of the nativity as 25 December. In the East, early Christians celebrated the birth of Christ as part of Epiphany (6 January), although this festival emphasized celebration of the baptism of Jesus.
Christmas was promoted in the Christian East as part of the revival of Catholicism following the death of the pro-Arian Emperor Valens at the Battle of Adrianople in 378. The feast was introduced to Constantinople in 379, and to Antioch in about 380. The feast disappeared after Gregory of Nazianzus resigned as bishop in 381, although it was reintroduced by John Chrysostom in about 400.
