No one is really sure when exactly it happened (or who started the idea), but this period of time eventually took on an obvious symbolic meaning to human beings. Many geographically diverse cultures throughout history have recognized the winter solstice is as a turning point, a return of the sun. Solstice celebrations and ceremonies were common, sometimes performed out of a fear that the failing light of the sun would never return unless humans demonstrated their worth through celebration or vigil.
It has been claimed that the Mesopotamians were among the first to celebrate the winter solstice with a 12 day festival of renewal, designed to help the god Marduk tame the monsters of chaos for one more year. Other theories go as far back as 10,000 years. More recently, the Romans celebrated the winter solstice with a fest called Saturnalia in honor of Saturn, the god of agriculture.
Integral to many of these celebrations were plants and trees that remained green all year. Evergreens reminded them of all the green plants that would grow again when the sun returned; they symbolized the solstice and the triumph of life over death.
In the early days of Christianity, the birth of Christ was not celebrated (instead Easter, was and possibly still is the main holiday of Christianity). In the fourth century, the Church decided to make the birth of Christ a holiday to be celebrated. There was only one problem - the Bible makes no mention of when Christ was born. Although there was some evidence to draw from, the Church chose to celebrate Christmas on December 25. It is believed that this date was chosen to coincide with traditional winter solstice festivals such as the Roman pagan Saturnalia festival in the hopes that Christmas would be more popularly embraced by the people of the world. And embraced it was, but the Church found that as the holiday spread, their choice to hold Christmas at the same time as solstice celebrations did not allow the Church to dictate how the holiday was celebrated. And so many of the pagan traditions of the solstice survived during the next millenia, even though pagan religions had largely given way to Christianity.
And so the importance of evergreens in these celebrations continued. The use of the Christmas tree, as we now know it, is generally credited to sixteenth century Germans, specifically the Protestant-reformer Martin Luther, who is thought to be the first to added lighted candles to a tree.
While the Germans found a certain significance in the pagan traditions concerning evergreens, it was not a universally held belief. For instance, the Christmas tree did not gain traction in America until the mid-nineteenth century. Up until then, they were generally seen as pagan symbols and mocked by New England Puritans. But the tradition gained traction thanks to German settlers in Pennsylvania (among others) and increasing secularization of the holiday in America. In the past century, the Christmas tree has gained in popularity, as more and more people adopted the traditon of displaying a decorated evergreen in their home. After all this time, Christmas trees have become an American tradition.
There has been a lot of controversy lately concerning the presence (or, I suppose, the removal and thus absence) of Christmas trees in schools. Personally, I don't see what is so controversial about it, as a Christmas tree is more of a secular, rather than religious, symbol. Joshua Claybourn quotes the Supreme Court thusly:
"The Christmas tree, unlike the menorah, is not itself a religious symbol. Although Christmas trees once carried religious connotations, today they typify the secular celebration of Christmas." Allegheny v. American Civil Liberties Union Greater Pittsburgh Chapter, 492 U.S. 573, 109 S.Ct. 3086.It does not represent a religious idea, but rather the idea of renewal that accompanied the winter solstice. One can associate Christian ideas with the tree, as Martin Luther did so long ago, but that does not make it inherently Christian. Indeed, I think of the entire Christmas holiday as more secular than not, though I guess my being Christian might have something to do with it. This idea is worth further exploring in the future, so expect more posts on the historical Christmas.
Update: Patrick Belton notes the strange correlations between Christmas Trees and Prostitution in Virginia.