The date of 25 December for Christmas is first securely attested in the fourth century CE, and historians have proposed two main explanations for its selection, neither universally agreed upon: one links it to the Roman winter solstice and the feast of Sol Invictus, celebrated on or near the same date, suggesting the Christian feast was positioned to supplant or absorb a existing pagan Roman festival; the other, sometimes called the calculation theory, derives it independently from the fixed 25 March date of the Annunciation, itself calculated from an older tradition associating that date with the crucifixion, on the reasoning that a great prophet's conception and death should fall on the same calendar day. Most historians regard the two explanations as not mutually exclusive, and some evidence supports each.
Modern critical scholarship, following the analysis Raymond Brown set out in his standard commentary on the infancy narratives, treats Bethlehem as the birthplace assigned by both Matthew and Luke primarily because Hebrew Bible prophecy, particularly a passage in the book of Micah, named it as the expected birthplace of a coming ruler from David's line, raising the possibility that theological expectation shaped the reported birthplace rather than the reverse.
None of this uncertainty is treated by mainstream Christian tradition as undermining the core claim that Jesus was born and that his birth is theologically central to Christian faith; the debate concerns specific historical details of place, date and surrounding circumstance that the two independent Gospel accounts describe in ways not fully reconcilable with each other or with the wider historical record.