Well, we have the Advent wreath and lights and calendars, and little hand-carved nativity scenes and things, before. It already looks very "Christmassy", but the tree is the highlight. Also, the tree has to last to at least Epiphany, and the heating would destroy it, if you put it up, too early.

They used to actually hang the trees from the ceiling. My mom said they did it because the houses were too small, to put them on the floor. Like this.