A great article from the website of Biblical Archaeology Magazine: How December 25 Became Christmas
The Bible offers few clues: Celebrations of Jesus’ Nativity are not mentioned in the Gospels or Acts; the date is not given, not even the time of year. The biblical reference to shepherds tending their flocks at night when they hear the news of Jesus’ birth (Luke 2:8) might suggest the spring lambing season; in the cold month of December, on the other hand, sheep might well have been corralled. Yet most scholars would urge caution about extracting such a precise but incidental detail from a narrative whose focus is theological rather than calendrical.
A good reminder: we need to be careful not to look for details that presumably were left out on purpose.