From Andrew McGowan of the Biblical Archeology Society:
On December 25, Christians around the world will gather to celebrate Jesus’ birth. Joyful carols, special liturgies, brightly wrapped gifts, festive foods—these all characterize the feast today, at least in the northern hemisphere. But just how did the Christmas festival originate? How did December 25 come to be associated with Jesus’ birthday?
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.
The extrabiblical evidence from the first and second century is equally spare: There is no mention of birth celebrations in the writings of early Christian writers such as Irenaeus (c. 130–200) or Tertullian (c. 160–225). Origen of Alexandria (c. 165–264) goes so far as to mock Roman celebrations of birth anniversaries, dismissing them as “pagan” practices—a strong indication that Jesus’ birth was not marked with similar festivities at that place and time.1 As far as we can tell, Christmas was not celebrated at all at this point.
This stands in sharp contrast to the very early traditions surrounding Jesus’ last days. Each of the Four Gospels provides detailed information about the time of Jesus’ death.