I remember reading somewhere that Jesus was probably born around October when shepherds in Palestine would be most likely to sleep outside with their sheep. Jesus may not have been born in December, and it certainly was not a "bleak mid-winter" with blankets of snow. Images of a frosty night are historically inaccurate ... but I don't mind them, because they're theologically correct.
The celebration of Jesus' birth was combined with winter festivals for sound spiritual reasons. The prologue of John's Gospel (which says nothing about Jesus' birth, yet is considered a classic Christmas text) speaks of Jesus entering the world as a light shining in the darkness. And that is what Christmas is about.
It is at the darkest, coldest time of year--in the time most bleak, when we feel closest to death--that we remember how God entered the world. Because he didn't just come for a pleasant holiday. He came because we needed him. Desperately. Christmas reminds us that God is with us in our darkest hour.