Many Western cultures used December 20th/21st/Winter Solstice as the "birth" of the new year, when the sun starts it's trek back with more daylight. Other cultures, especially some ancient Middle Eastern ones, used the highest point of Sirius, which coincides with January 1st. That was the date the Church settled on when creating a new calendar, possibly because it was popular 'pagan' dating in a lot of the surrounding cultures and the Church was actively seeking to work them in.
Funny how much of our secular world gets it's roots from the religious which gets it's roots from the pagan which gets it's roots from secular planting times, etc. ;)