>> So that means you have an answer for what shepherds were doing outside in the fields at night, with their flocks of sheep, at the end of December, in winter? <<
I do. It’s called the Roman Warm Period.
Look, the date of Christmas was inferred, but on a pretty decent basis, not the pagan In Sol Invictus, created centuries after Christmas to superecede it, and not Saturnalia, which simply wasn’t related and not at the same time.
The Book of Maccabees stated that God dwelt in his Temple again after the Feast of the Dedication, which was on the 25th of the month closest to December (albeit admittedly, tending to be a few days early.) Jesus identified himself with the Temple, so selecting the Feast of the Dedication to celebrate his birthday seems sound.
Also, however, there was an ancient tradition that great prophets were conceived into the afterlife on the day they were conceived into the world. Thus, since Jesus died on March 25th, according to tradition, that day is celebrated as the Feast of the Annunciation, celebrating his conception. Doubtlessly, then, the ancients counted forward nine months to December 25th.
Neither of these prove a birthdate of December 25th; they merely demonstrate that it was a reasonable supposition. The ancients who made that supposition would well have been familiar if weather issues argued against the plausibility.
Chrysostom in the 4th century argued for adopting the Roman date for the feast of Christmas. One of his arguments was that the Romans derived the date from the census records.
We have no way of proving that was indeed the case, but it is certainly possible and cannot be discounted offhand.