In my family it used to be tradition that the tree was put up on Christmas Eve. I remember there was always some struggling to saw off what was needed to make the tree stand straight, then we would get out the old ornaments.
I know that not everyone has a Christmas tree, though ...
Penny: Hey Sheldon, are you and Leonard putting up a Christmas tree?
Sheldon: No, because we don't celebrate the ancient Pagan festival of Saturnalia.
Penny (confused): Saturnalia?
Howard: Gather round, kids! It's time for Sheldon's beloved Christmas special.
Sheldon: In the pre-Christian era, as the winter solstice approached and the plants died, pagans brought evergreen boughs into their homes as an act of sympathetic magic intended to guard the life essences of the plants until spring. This custom was later appropriated by Northern Europeans and, eventually, it becomes the so-called Christmas tree.
Howard: And that, Charlie Brown, is what boredom is all about.
The Big Bang Theory, USA, 2007 -