> hey deliberately put it at a time where many northern folks had their own traditional end of the year
For most of history, Christianity is a mediterranean religion. There would be no reason to choose to put it at a time for Northern Europeans.
For example, I am the descendants of the indigenous Christians of India. The historical celebration of Christmas is called Denha, which is the native name for Epiphany, for which there is widespread textual evidence (corroborated by similar celebrations elsewhere), and which was celebrated at the usual time (Jan 6). There are numerous councils of the Church of the east that confirm this as well. Why would they possibly care what Scandinavia did. The simple truth is that this is a really old time to celebrate the birth of Christ, for reasons lost to history. Unlikely that the Northern Europeans, who were catechized many centuries after Christ played a large role in it. We have records of the date in Asian church councils well before the Northern Europeans were even contacted people.
The reason why Northern European traditions (trees, santa claus, snow, etc) became popular is due to the migration of Northern Europeans to America which is today synonymous with 'Christian' nations. even in Europe, the indigenous Catholics of South Europe don't always celebrate in this way.
But there are universals: sweets, gift-giving, celebration, stars, these are all common.