The early church did not celebrate the Nativity at all, Origen lists only Preparation, Passover, Pentecost. Clement of Alexandria, writing after 192, offers competing dates: November, January, April, May. Hippolytus, around 204, is the first to say December 25, reasoning that Jesus was conceived March 25, the equinox, and born nine months later, under the rabbinic logic that the perfect die on their conception date. The De Solstitiis tract then aligned all four feasts to the solar year: conception at the spring equinox, John's birth at the summer solstice, his conception at the autumn equinox, Christ's birth at the winter solstice.
The solar mapping was intentional and preached: John the Baptist, born at the summer solstice, when light begins to shrink, says of Jesus he must increase, but I must decrease, and the church aligned the calendar to that verse, light diminishing from John's feast, growing from Christ's. Cyprian called Christ the true sun; the Tomb of the Julii mosaic, around 300, pictures him as Helios driving the chariot. The church did not hide the solar architecture. It advertised it, in sermons, against the pagans, as a victory.
The claim: Christmas demonstrates that sacred dates are infrastructure, not history, the winter turn of light was always going to be someone's feast, and the fourth-century church simply built the best-durability version on the lot, calculation theory and solar appropriation being two descriptions of the same construction project. Verdict: the record supports the cynicism and transcends it, the feast's endurance suggests the solstice slot really does answer something human. Boniface's letter to the Pope, 742, complaining that Romans still parade pagan-style at New Year, is the receipt that the lot was never fully cleared.
If the date was calculated, not recorded, what else about the feast is architecture?