It is amazing how often academics concentrate on apocryphal works like the Infancy Gospel while neglecting far better sources. The question of when one should celebrate the birth of Jesus is obviously a question related to the development of the liturgical calendar. Where should one turn first if you want to know about liturgy? I’d recommend looking, not at the apocrypha, but at lectionaries. Unfortunately, our earliest surviving lectionary is from the 4th century, and it’s only fragmentary. I suspect, though, it would be a fair enough guess to say that, sometime in the 2nd century (when the list of Saturday and Sunday readings began to be developed) someone selected a time in late December or early January for the readings from the first chapters of Matthew and Luke. While the “new year” is reckoned differently on the Jewish calendar, the start of a new Roman year would have been an appropriate place to begin the cycle of Gospel readings. And once the late December/early January readings about the birth of Christ were fixed, celebrating Christ’s birth near that time was a natural continuation.
Excellent, excellent point. It needs to be treated as a liturgical question as well as a historical one.
Do you know if the text of that 4th century lectionary is online anywhere? We have a 4th century martyrology, and Christmas is on it.