Christmas Tree in Germany (late 1700s, by Joseph Keller)
Because Christmas is related to old celebrations of the winter solstice, evergreen trees have always been popular decorations in northern countries at Christmastime. They're green when everything else is dead and white. In the time of the Roman Empire, people sometimes hung small bits of metal from trees at this time of year - but they were living outside trees, not cut ones. In the Middle Ages, in Europe, people sometimes hung apples from trees for Christmas. But people only started to cut pine trees down and bring them inside houses in 1531 AD, in northern Germany. Probably these Christmas trees were a Protestant substitute for the nativity scenes and mangers in Catholic houses.
These earliest Christmas trees were plain; they didn't have any decorations on them. By 1605, though, some people at least were decorating their Christmas trees with paper roses, apples, and sugar candy (which had just been invented). In 1610, rich people began to put thin strands of real silver tinsel on their Christmas trees. By 1650, they were hanging dolls and maybe other presents from their trees too. Parents or servants then shook the trees so the presents and candy fell off and the children could grab them.
American Christmas Tree (about 1812 or 1819)
As Christmas trees got more popular in Germany, people in other countries began to want Christmas trees too. People thought of their Christmas tree as a German thing, and they bought their best Christmas tree decorations from German stores.
Queen Victoria's Christmas Tree (1840s)
Beginning in the early 1800s, as more German people moved to North America, they brought the idea of Christmas trees with them. Americans who weren't from German families didn't have Christmas trees.
In 1841, though, the German Prince Albert, who was married to Queen Victoria of England, had a Christmas tree in Windsor Palace in England. Newspapers in the United States printed the picture of Queen Victoria's family around their tree. Everyone admired young, pretty Queen Victoria, and many rich families in the United States began to have their own Christmas trees, even if they weren't from German families.
By the 1880s, as more Italian immigrants came to the United States, having a Christmas tree became a way to show that you were a patriotic American, and not an Italian who had a nativity scene.
In the course of the 1900s, these two traditions mixed, and by 1982 even the Pope in Rome had a Christmas tree in his Vatican palace.