Two Rival Explanations for a Date the Gospels Never Give

No Gospel states a date for Jesus's birth, and the earliest surviving source to name December 25 explicitly is the Chronography of 354, a Roman document also known as the Calendar of Filocalus, whose civil calendar section records games for Sol Invictus on the same date and whose separate martyrs list records Christ's birth at Bethlehem on it as well, the earliest point at which the date is attested for either observance at all.

Historians offer two main explanations for how the date was fixed, and neither is settled. The history of religions theory holds that the date was chosen in relation to the existing Roman festival of Sol Invictus, the unconquered sun, instituted on the same day by the emperor Aurelian in 274 CE, either in deliberate contrast to it or by natural association with the winter solstice both dates sit near. The competing computation hypothesis argues the date was reached independently, through an early Christian tradition holding that Jesus was conceived on March 25, itself calculated from a belief that great figures die on the same calendar date as their conception, with birth following nine months later on December 25 without reference to any Roman solar festival at all.