a thousand years ago in Scandinavia the new day would start at 6pm, effectively at sunset, which is also why Christmas in Scandinavia is on the evening of the 24th instead of the 25th in the morning.

Or so the legend goes. I'm pretty sure that the ancient winter solstice feast had something to do with it as well. In Scandinavia the word for christmas is still some variation of the ancient "Yule" meaning wheel in old norse.