what is now December 21 or 22. (Winter Solstice)
That point isn't really arbitrary, as on that day the Earth is in such a position that the Northern Hemisphere (which would have been the dominate consideration at the time) has it's shortest day. (that is, daylight is the least of any day of the year)
I think originally, this was the case with the prior Julian Calendar, but because it was inaccurate, the drift pushed it 10 days later.