I was listening to a very old Christmas CD the other day and it had "Go Tell It on the Mountain" on it. One line in the recording explicitly mentioned Jesus being born on December 25. I suspect most people who would read this know that is not the real date, just the date we celebrate.
This piece in BAR is very interesting (I think I got the link from Allan Bevere). It suggests that some early Christians before Augustine started using the dates January 6 and December 25 because they were nine months after suggested dates for Jesus' crucifixion, the assumption being that Jesus was conceived and died on the same date. It argues that some Christians were already celebrating Christ's birth on Dec. 25 prior to Constantine.
The birth narratives in Matthew and Luke are quite different. They do have a common core. They both agree that Jesus was born in Bethlehem and ended up in Nazareth as a child. They both agree that Mary was a virgin when the Holy Spirit brought about her conception. That's about it.
Each gospel emphasizes in its presentation the kinds of things we would expect them to, given their emphases elsewhere. Matthew has overtones of Moses, which fits since Jesus gives the fulfilled understanding of the Law. As Pharaoh killed Israelite babies, Herod kills the boys of Bethlehem. Moses came out of Egypt; Jesus comes out of Egypt. Matthew connects Jesus' birth to the OT. The Magi remind us that the gospel is for the Gentiles too and that Jesus would become a king.
Luke gives prominence to women in the story of Jesus' birth, as he does elsewhere. Lowly shepherds come, fitting with Luke's emphasis on the poor and lost sheep. The temple features prominently, as it does elsewhere in Luke's presentation.
I suppose that different Christians take different things from Christmas. For me, Christmas is the incarnation. It is God coming to earth. It is the beginning of salvation for the whole world.
For me it is also Jesus being born amid scandal, yet another identification of Jesus with the lost sheep of the world. Was he ridiculed as a child for being born before his parents were married? Did his younger brother James think himself a little purer than his older brother?
For Matthew, the birth story revealed that Jesus birth was in continuity with who he came to be in life later. This was the expectation of ancient biography. So there were signs he was a king from the start. Did Luke want to balance this out, if Luke knew Matthew?
Interestingly, the New Testament does not make much of the virginal conception. [PS] Even Matthew and Luke themselves never mention it again and they give no interpretation to its meaning. It does not seem required for Jesus to be divine, since he is not half man and half God but fully human and fully divine.
So what do you celebrate at Christmas? I celebrate "the beginning of the gospel" (Mark 1:1), even though Mark is referring to Jesus' baptism.
[PS] We informally refer to the virginal conception as the virgin birth. Catholics do believe Mary miraculously remained a physical virgin even in birth, as well as that Joseph never had sex with her thereafter. Most Protestants believe Mary went on to have other children--James, Jude, etc...