I just came across an excellent article about the date of Jesus' birth by Paul Meier, a prominent New Testament scholar. Since it is rather long and technical, I will summarize it here.
We celebrate Jesus' birthday on December 25, but it is quite unlikely that he was born on that day. That date was picked out in the fourth century, most likely as a replacement celebration for the winter solstice or other pagan holidays.
Paul Meier suggests a birthday in November. This is based on two pieces of data. First, Luke's nativity story begins with the account of Zachariah's service in the Temple at the assigned time for his priestly division (Abijah). A few weeks later, his wife conceives; six months later, Mary conceives; nine months after that, Jesus is born. Since Zachariah's priestly division served in late July to early August (according to some educated guesswork based on early rabbinic documents), Jesus would have been born in November.
By itself, that would not be very strong evidence. However, that date is backed up by the very earliest reference to Jesus' birth date. Clement of Alexandria, one of the church fathers, wrote in AD 194 that Jesus was born 194 years, one month, and 13 days before the murder of emperor Commodus - a significant event that occurred on December 31, AD 192. (By the way, Commodus is the same emperor fictionally depicted in the movie Gladiator). Although Clement seemed to get the year wrong, he may well have had the correct day - November 18.
Many people already know that Jesus was, ironically, born BC. The sixth-century monk Dionysius Exiguus, inventor of our BC / AD system, made two errors. First, he was off by about four to seven years; and second, he forgot to include a year zero. Our calendar goes directly from 1 BC to AD 1, which throws off computations.
How do we calculate the correct year? Herod the Great, who figures prominently in Matthew's birth account, died in March of 4 BC (a date pinned down by a lunar eclipse recorded in Josephus' history). Jesus must have been born before then. He may have been born as early as 7 BC, but several details suggest that 5 BC is the most likely year. If so, Herod died only four months after his attempt on Jesus' life.
So Jesus' birthday, by Paul Meier's cautious estimate, is November 18, 5 BC. Any one planning on moving your Christmas celebration to before Thanksgiving? If you do so this year, be sure to put 2012 candles on the cake (AD 2008 + 5 BC - 1 for Dionysius' mistake).
In the end, the day or even the year of Jesus' birth is not certain, like the birthdates of most other ancient people. Nor is that date terribly important. But it reminds us that Jesus is a real historical person; his life can be investigated using the normal methods of historical inquiry. He is not merely a convenient, timeless myth or an artificial object of faith.
The picture: the Adoration of the Magi, from The Brick Bible.