In the classic episode " Busy Christmas" first shown on December 19, 1956, the Nelson's Christmas Tree is in the den, as Harriet wanted to suprise Ozzie as to its location. In the very next episode " The Day After Christmas" first run on December 26, 1956, the Christmas Tree is now in the main living room. Now I have come up with some reasons for this, see if you can also speculate as to why it has been moved:

1) There was an earthquake on Christmas Eve and the tree shifted from the den to the living room.
2) Harriet never intended for the Christmas Tree to be permantently in the den, since it was there to surprise Ozzie who never got around to buying a tree.
3) It was easier to film the family in the living room as the room was larger than the den and the family ate so much on Christmas Day, they were all five pounds heavier.
4) They had two Christmas Trees that year, one in the den and one in the family room.