Also important to remember most of the traditions about it are pagan. Christ wasn’t even born in December. They just decided to celebrate it then to coincide with the existing solstice traditions. Many places celebrated the solstice as a new beginning, the days were now getting longer, and people needed a pick-me-up in the dark season. It was often one of the biggest annual celebrations. So it was co-opted with one of the most obvious Christian signs matching the theme, Christ’s birth being the beginning of the end of a time of darkness. First by giving a new meaning to Saturnalia, then adding more bits from other regions they were trying to convert.

Not just Saturnalia, that was just the first. More was incorporated from various traditions as they sought to convert them. But yeah, that was sort of the foundation the rest was laid upon.