Winter solstice
In the Roman Empire, 25 December (in the Julian Calendar) date was celebrated as being the day when the Sun was born, the birthday of Sol Invictus, as can be seen in the Chronography of 354. This date corresponding to the date of the Winter solstice. Early Christians considered this a likely date for their saviour's nativity, as it was commonly held that the world was created on Spring equinox (thought to fall on 25 March at the time), and that Christ had been conceived on that date, being born 9 months later on Winter solstice.