Those who argue for a September birth usually use as their rationale that since some of the important events in the life of Christ and the early Church are associated with important high Jewish holy days, then Jesus' birth should be as well. And thus they try to argue that Jesus must have been born during Sukkot, the Jewish Feast of Booths/Tabernacles.
The problem with this theory is that the Gospel stories clearly state that Joseph and the very pregnant Mary traveled from Nazareth to Bethlehem, and that Jesus was born in Bethlehem. Note that Joseph is present. This is a huge problem because Sukkot was one of the three pilgrim feasts in Judaism, in which all adult Jewish males were obligated to come to Jerusalem to observe the feast. These feasts are Passover, Shavu'ot (aka "Pentecost"), and Sukkot. This is also why, for example, in Acts ch. 2 there are Jewish pilgrims from all over the place gathered in Jerusalem, and so when the disciples begin speaking in various languages the pilgrims recognize the languages as including their own.
According to the Gospel of Luke the reason Joseph took the family to Bethlehem was because of a census, where Jewish men were to go the place of their ancestry. It would make little sense for Roman officials to have Jewish men travel to the places of their ancestry at a time when they were obligated by their religion to be in Jerusalem.
That is to say, given the data in the Gospels, it is simply not feasible for Jesus to have been born at this time.
Additionally, the usual claim that Jesus couldn't have been born during the winter is an oft-toted claim, that there wouldn't have been any shepherds with flocks as the nativity stories say. Except that the fields outside of Bethlehem have been used for millennia for the purpose of shepherds over-wintering the flocks. Even into modern times shepherds have brought their flocks down from the hills into the fields outside of Bethlehem to over-winter them. So it is precisely during winter when we would expect a lot of shepherds and their flocks in the fields outside of Bethlehem.
Some other interesting information: The Jewish kohanim (priests) were divided into several divisions, and priests were rotated throughout the year according to these divisions. Each division served twice during the year, and also all the divisions were supposed to be present at Passover. According to the Gospels Zechariah, the father of John the Baptist, was of the division of Abijah. One of the times of the year in which the division of Abijah served was in autumn, September-October-ish. If Zechariah was serving at this time, then that would make Elizabeth's pregnancy with John around this time. According to Luke Elizabeth was six months pregnant when Mary conceived Jesus, this would put Jesus' conception in March-April. And, wouldn't you know, nine months later would be December-January.
Now, none of this says for certain that Jesus was born in the winter. But the whole "Jesus couldn't have been born in December" argument doesn't hold the kind of water some think it does. The data we have from the Gospels themselves provides enough to make a December-January birth as, at least, not only possible but even plausible.
-CryptoLutheran
