The historical origin of Christmas is unclear.
So wrote John Sweet in his Introduction to Using Common Worship: Times and Seasons (All Saints to Candlemas) (p. 6).
One suggestion is that it was ‘the Christianization of a pagan festival’, namely, the ‘birthday of the unconquered Sun’, which was celebrated on 25 December. But Sweet prefers an alternative explanation:
It seems more likely that the true clue is the date of Passover (25 March in the West, 6 April in the East) and Jewish identification of end with beginning: seeing the conception of Jesus as one with his Paschal victory and dating his birth nine months later. Such temporal coincidences were commonplace for the Jews, as they were for poets like T. S. Eliot and John Donne (see his poem ‘Upon the Annunciation and Passion falling upon one day, 1608’).
If this is correct, it does help to emphasise the unity of God’s redemptive work in Christ: incarnation, death and resurrection are intricately connected.
It also makes me wonder why we don’t make more of the Annunciation? It’s not as if the incarnation happened when Jesus emerged from his mother’s womb, after all: the unborn baby was God incarnate from the moment of conception.
However, calendars are messy, and even if there was originally a connection between Passover and the Annunciation, it has been obscured by the subsequent dating of Easter, and even further by our rules for ordering the Christian year. In fact, if the Annunciation has the audacity to fall between Palm Sunday and the Second Sunday of Easter, it is transferred to the following Monday!
Perhaps it’s time to reform our calendars and straighten everything out…?
(Update: James Patrick published a blog post on this same topic a few days ago: The Jewish Origins of the Date of Christmas.)
(Update 2: Tom Holland has also written on this topic: The myth of ‘pagan’ Christmas. ‘Christian scholars, drawing on similar traditions, came to believe that Jesus had died on the anniversary of his incarnation. And the date of that anniversary? First in Carthage, and then in Rome, it came to be identified with what, according to the Roman calendar, was 25 March. Then, once that particular date had bedded down — and operating on the assumption that Christ had been born nine months after his conception — it required only a simple calculation to arrive at the date of his birth. By the 4th century, 25 December was coming to be enshrined across the western half of the empire as the anniversary of Christ’s birth.’)