Christmas trees...a misnomer, another custom stolen from pagans by Christianity.
Where did the custom of decorating evergreen trees originate and why? It depends on who you ask.
Protestants may tell you it was all Martin Luther's idea. The story goes that Marty was traipsing through the woods one snowy evening and was overcome with the beauty of the snow and moonlight glistening from the evergreens. So he brought a small tree into the house to replicate the scene for his children, adding candles to simulate snow glistening on the branches, or the starlight at Jesus' birth, or the general beauty of God's creation, or the light of truth or something. It's a nice thought (whichever rationale you prefer), but it has no basis in fact, no evidence at all.
Catholics may tell you the Christmas tree was all St. Boniface's idea. He lived in the 600-700s and is the patron saint of Germany (and brewers, incidentally). He was an Anglo-Saxon Christian missionary in Germany. There is a legend that in order to disprove the existence of the Norse gods who were worshipped there, he chopped down an oak tree that was sacred to the pagans. It was known as Thor's Oak. (Thor is also known as Donar or Donner, incidentally, just like the reindeer.) Not only did Thor NOT smite St. Boniface on the spot (thereby casting doubt on the existence of the Norse gods), but also there was apparently in the roots of the oak tree a fir tree growing. Thus, the Catholics say, the evergreen Christmas tree represents the triumph of Christianity over the pagan gods. But modern Christmas trees didn't really become popular until many centuries later, and this story doesn't mention decorations or lights or bringing the tree into the house.
Somewhere in Latvia/Estonia, they also claim the Christmas tree origin because in the 1400s-1500s trees were cut down and then erected at guild halls and/or in the town square, where people sang and danced around them and then burned them. They also would decorate the trees with apples, nuts, dates, pretzels, and/or paper flowers. (However, no lights are mentioned.) I am one of those types who like to ask "But WHY?" and I haven't had my question answered very well by the interwebs. It might be so the kiddies could take the tasty treats; it might be because apples are another fertility offering to various Norse gods or Celtic woodland sprites; or it might have something to do with Christian morality plays involving Adam and Eve and the forbidden apple from the Tree of Knowledge.
Evergreens in general have been a popular decoration near the winter solstice since ancient times. When other plants are dying yet evergreens live, they are naturally a powerful symbol of life triumphing over death or of survival in the harshness of winter. Ancient Romans brought evergreens (boughs of holly or ivy) into their homes to decorate during the Saturnalia festival. Roman pagans also decorated trees with silver, gold, and possibly candles, though this was likely done outdoors. Egyptians brought palm tree branches into their homes during their solstice festivals for the same reasons, having to do with resurrection and fertility, to remind us that life can survive in the darkest days of winter.
Yet again the holiday traditions of various cultures and religions seem to blur, and everybody picks a rationale that is most convenient to their way of thinking. I haven't even mentioned the Druids and the mistletoe that represents the semen of the god or the holly berries that are the menstrual blood of the goddess. Holly, with its prickly leaves, is also meant to trap evil spirits like magic fly paper. You think I'm making this up, right? No, but I might as well.
My research seems to suggest that you can simply celebrate any holiday at any time with your favorite ritual so long as you can make up some story about why it might be relevant. Therefore, I think I will decorate my Multiple Ambiguous Winter Holiday Tree with apples to symbolize the Mac that I'm using to figure this stuff out; pretzels to symbolize the knots of convoluted and inter-twisted origin myths; and nuts to symbolize how crazy it is. If I then eat these decorations at New Year's, with luck, they will "fertilize" creativity in my blog posts next year, or perhaps produce an Epiphany!