Here’s what I read a while ago how we get Dec. 25. In the gospel of Luke we read of Zachariah serving in the temple and getting a visitation from the angel Gabriel foretelling him of his son to be born. It is known by his division that his time of service would have been in late September. 6 months later Mary is visited by Gabriel. The annunciation and conception of Jesus is said to be March 25 followed by her visit to her cousin Elizabeth pregnant with John the Baptist. In days of old this date was honoured and celebrated even more than Christmas. Dec. 25 is nine months later.
That was my understanding as well. John as Elijah in Spirit was supposed to precede Jesus to fullfill scripture as the announcer of Christ’s coming. We know that Elisabeth was in her 6th month when Mary, newly “encent” with Jesus, visited her. Since we know about the time John’s father would have been doing his priestly service, we know when Elizabeth would have conceived John and then do the calculations. Infant gestations are about 36 to 40 weeks. Orthodox traditions hold that the Birth of Christ was to be some 11 or 12 days later(January 6 or 7) due to the Gregorian Calendar based traditions but that would accomodate nearly a 40 week gestation verses 36 to 38 week(9 month) one.
Hence December 25 is certainly plausible. I think there was a question about what office John’s father actually held and what month he was actually doing his service. Some have argued that since there flocks in the field by night(too cold in December some have speculated) that Christ’s birth must have been mid march to April and John’s birth was actually around the September trumpet/feasts periods. 6 months later puts Christ’s birth in mid late March to mid April.(depending on a 36 to full 40 week gestation) Also Jewish months were 28 days with corrections thrown in some years due to incongruencies between the lunar calendar system and the seasons. Now whether the gospel sources were written with the Julian Calendar in mind by the writers or the Lunar Calendar of the Jews I can’t say.
It would add some poignancy to Anna’s prophecy about Christ’s dying for his people which we know was around mid April. Did he die around his birthday? Who knows...does it matter?