Long before Christianity existed people held evergreens to be a special (especially the trees) since they did not die in the winter as other trees and plants did. The evergreen tree had special meaning for many people as it reminded them of the spring and summer that would soon bring all sorts of living plants and trees alive again. Many ancient folks hung boughs of evergreen over their porches and other parts of their homes.
In the northern hemisphere the winter solstice occurred on December 21rst or 22nd and was an important day for many ancients. These folks believed the sun was a God and that winter came basically because the sun God became weak and sick. Again, the evergreen reminded them of the hope of spring and summer to come and they used boughs of evergreens to decorate a bit.
Early Romans celebrated the solstice with a feast called Saturnalia and used evergreens as decorations inside their homes, palaces, and public buildings.
Druids thought that there was something mystical about the evergreen and decorated their temples with evergreen as a symbol of everlasting life.
The first actual Christmas tree is credited to the Germans in the 1500's. Finally - we have a Christmas tree up! Martin Luther is credited with actually putting lights on the tree and this started a tradition of fixing the tree up a bit to make it look a bit more more festive.
But, even by the 1800's Americans still saw Christmas trees as an oddity though they finally started popping up in Pennsylvania by German settlers who had brought the tradition with them from their homeland. Still, even by the mid 1800's most Americans held the Christmas tree as a pagan symbol and would have nothing to do with it.
It wasn't until the 1890's that Americans took to Christmas trees. That is surprising to me as I would have thought that Christmas trees in America took hold by the early 1800's. Funny what you find out when you do a little research.
By the early 1900's Americans had bought into the notion of the Christmas tree though it was decorated primarily with homemade items and most people did not put their trip up until Christmas Eve - often after the children had gone to bed as the Christmas tree, itself,, was considered something of a Christmas gift. Some believed that Santa (or St. Nick) brought the tree.
Well, there you have it. A very brief history of the Christmas tree that we take so much for granted today.
One of the many reasons I have no problem celebrating the season while being utterly non-religious. The party predates most of our contemporary gods. Like it or not, the tree, the mistletoe, even "Yuletide" itself = pagan. (no, I'm not pagan either :)