My friend Zuly has a great piece on the origins of modern winter holiday traditions, including holly wreaths and mistletoe. For those who choose not to follow the above link, I have excerpted the following:

So the next time you’re looking at a wreath, or a decorated tree, or placing some icicles on the tree, know that you are connecting with traditions that go back thousands of years, and by doing so, are connecting with your ancestors. This is a time of rebirth, as the long night gives way to the new dawn, and as such, let the magic of the season imbue you with an inner warmth to help you in the coming days against the long, cold winter.