Disagreeable's first posts have errors. first, Christmas began to be observed on Dec. 25 around AD 325. The Nativity scene was said to have been begun by St. Francis in 1223.
All people were aware of the changes in seasons and the beginning of lenghtening of days, and would have been a natural place to put the birth of Christ in their yearly celebrations of Christian events, because the Birth of Christ was a new beginning in Religious history. Not merely a replacement of Sun worship.
As for Americans not celebrating Christmas before 1800, I am not suprised at some of those Christians as they were very strict and didn't believe in celebrating anything! However there were some Christians, and maybe Catholics in particular who would have noted the event in a Mass of celebration. (Hence Christ-Mass)