Dies Natalis Solis Invicti, 25 December

Dies Natalis Solis Invicti, the birthday of the unconquered sun, is attested for December 25 in the same document that gives the earliest clear attestation of a December 25 Christmas, the Chronography of 354, whose civil calendar section records games for Invictus on the date. The cult of Sol Invictus itself was promoted to particular prominence as a state cult by the emperor Aurelian starting in 274 CE, though whether Aurelian specifically instituted December 25 as the god's festal date, or whether the date's association with Sol predates him and he simply elevated an existing observance, is not settled by the surviving evidence.

The relationship between this festival and the Christian Christmas dated to the same day remains a genuinely disputed question among historians of religion rather than a resolved one. Some scholars read the shared date as evidence that the Christian feast was positioned deliberately in relation to the established solar festival, while others note that the Chronography of 354 is itself the earliest firm attestation of both dates together, with no earlier source confirming which observance, if either, came first or influenced the other, a genuine gap in the evidence rather than a settled sequence obscured by later retelling.