• December 21: winter solstice (Northern Hemisphere). The day with the least daylight (the most daylight in the Southern Hemisphere).
• December 25: Christmas. Influenced by the Saturnalia, which in turn was about the winter solstice.
• January 1: the first day of the year. Dates from the beginning of Roman consulships. These were probably related to the Saturnalia (but I can’t find a source concretely backing this).

My proposal: unify these dates by shifting the calendar by ten days, so that Christmas and New Year’s Eve fall on the winter (summer) solstice.

In the Northern Hemisphere, the last day of the year will be the shortest (and the longest in the Southern Hemisphere).