An old-fashioned Christmas, that's what we all say we want--but Christmas trees are part of everybody's definition of a "good, old-fashioned" Christmas, and they are a relatively recent development.
Nobody had Christmas trees in the Middle Ages. They certainly brought trees into the house, but it was chopped-up trees for firewood (preferably aged hardwood), not a semi-living evergreen. If you think about it, it indeed strange to cut down a live tree and bring it inside in order to express one's Christian piety. There are all sorts of possible pagan overtones one can imagine about the renewal of life and hope at the darkest time of the year.
The medieval-favored plant for Christmas was holly. Holly keeps its leaves green throughout the winter, as a symbol of rebirth, and the thorns and the red berries were seen as symbols of the Crucifixion--the thorns for the Crown of Thorns, the berries for drops of blood. Wait, you say, these should be Easter symbols, not Christmas symbols. But medieval Christians always thought of the beginning of Jesus's story--his birth--in terms of its end. He was born to die. What else do you think the myrrh was doing? This gift from the Wise Men was an unguent used in embalming.
The first definite appearance of Christmas trees was in Germany in the late eighteenth century. The story was that Martin Luther, over two hundred years earlier, had seen stars through the branches of an evergreen and been inspired to bring the tree indoors and light it with candles, but this story has its doubters.
Both England and the US first adopted trees during the nineteenth century, independently inspired by the Germans. Queen Victoria, married to a German (Prince Albert), was apparently the first to have a Christmas tree in Britain, though the well-to-do British soon followed suit. Originally trees were for the upper crust. The beloved story "A Christmas Carol" by Charles Dickens (the story with Scrooge and Tiny Tim) has no Christmas trees.
In the US, wealthy east-coast families were caught between, on the one hand, wanting a simpler, more "traditional" Christmas in a nineteenth century that was increasingly turning to factory-produced goods and commercialization, and, on the other hand, wanting to "make this the best Christmas ever" for their children. (The focus of Christmas had already shifted from the drunken revelry of earlier times to the child-centered celebration of the home.) Christmas trees met both these needs.
The first American Christmas trees were small, table-top trees, on whose branches were hung small presents like a toy boat or a candy cane. A semi-living tree (or top of a tree, now cut and brought indoors) was certainly nothing like a factory. And the children would, it was hoped, be very excited to see their gifts in a new arrangement (even if they were factory-produced).
For more on the history of Christmas celebrations, see my essay, "Contested Christmas."