I've read about how, as the Julian and then the Gregorian calendars were adopted, different nations or regions celebrated the official start of the new year on different calendar dates, which gradually standardized to Jan 1st over the centuries. But where and why did Jan 1st get established 10-ish days after the winter solstice? Why not on the solstice itself, or some other natural temporal milestone? I suppose the more general question is why the Julian calendar places the solstices and equinoxes on the 21st/22nd of their respective months and not closer to the precise beginning or end of the month. The ancient Romans were perfectly aware of the precise time of the solstice, so why the choice to offset the change of the calendar month/year by several days?