I always insist that before Christmas dinner, everyone changes into "smarter" clothes. As long as it's a bit special- I'm happy.
This does mean, however, my daughter will be wearing a fully netted purple satin ballgown and my son will be wearing a black kilt (he's a goth and think it looks cool- I think he looks like he's borrowed his sister's school skirt! I wouldn't tell him that though- as at least he's making an effort).
Does anyone else dress up for Christmas dinner?