Many people will say that December 25 (or January 6, if you are Eastern Orthodox) is a made-up date for Jesus’ birthday. They will say that Christians chose the day, not because it was Jesus birthday, but because it was the same time as Saturnalia. The idea is that Christians picked the date either to replace the pagan feast day, or to be able to celebrate something on that day in order to evade persecution. So, when was Jesus really born?
In Luke’s gospel, there are some hints at the real date of Jesus’ birth. Luke is the gospel writer who is most detail oriented, and the most likely to tie events to secular history. For example, in Luke 2:1-3, we know that Joseph and Mary went to Bethlehem as a result of a census. If we knew from Roman history when the census was, we would at least be able to place the date of Jesus’ birth within a season. Lacking this, however, we can begin to work backwards.
The people at that time believed that it took nine months (270 days) from conception to birth. This is not exactly right (at least these days), but it is fairly close.
Luke 1:26-38 records Gabriel’s message to Mary. There are two chronological clues here. One is that this event happened “in the sixth month” (v. 26). I have two commentaries (William Hendrickson and Matthew Henry) that say this is referring to the sixth month of Elizabeth’s pregnancy, but it could also mean the sixth month of the year. The other clue is that Elizabeth is in her “sixth month” of pregnancy when Gabriel spoke to Mary. For this to work, we need to assume that Mary conceived very shortly after Gabriel appeared, it is sixth months from Elizabeth’s conception of John the Baptist to Mary’s conception of Jesus.
So we know that John the Baptist was conceived 15 months before Jesus was born.
If we continue backwards, we note that John the Baptist’s father was Zecharias, a priest of “the division of Abijah.” This means almost nothing to the modern reader, but for a reader in Luke’s time this meant a lot. There were many priests by this time, and they had to take turns ministering in the temple. If you knew which division a priest came from, you could figure out when he served at the Temple. It was believed that a priest ended his service in the Temple at the end of a Jewish calendar year.
There is another assumption at work here. The calculation assumes that Zecharias, shortly after his vision, went home, and that John the Baptist was conceived shortly thereafter. This is not a bad assumption.
So if we follow these dates, working backwards from December 25, we get:
Gabriel appears to Mary (March 25)
Angel appears to Zecharias (September 25 of previous year).
There is an ancient church document (“De Solstitiis”) which works with the assumption that Zecharias completed his term of service in the Temple on this date (which is also when the Jewish year changes over.)
Here are some notes from a lecture by Dr. Jack Kinneer (Origins of Christmas) that explain how and when Christmas originated, and how the dates were calculated. This focuses on how the date was derived using biblical data.
Here is another article (Calculating Christmas, by William J. Tighe) which looks at other ways that the date of Christmas was calculated.
In conclusion, the date of Christmas that we celebrate may not be right, but it was a good educated guess by teachers in the Ancient Church. It was not a date picked because of its association with a pagan holiday. (In fact, the reverse may be true.) To get this date, they used information from the Bible, plus knowledge of Jewish practices, plus some assumptions that were common at the time.