Start out in a onesie until mid morning then change. Used to be Sunday best but as I'm cooking it's more practical to go the Christmas jumper route. Just wear what makes you comfortable.
We go to family for Christmas Day and I like to be comfortable but a little bit sparkly. So I shall probably wear black trousers, with a dark sparkly top or a Christmas top if I find one I like and some Christmas slippers. Maybe a glittery brooch or a flashing necklace from the Christmas shop!
I would feel strange in jeans and a tshirt, but uncomfortable too in a smart dress or suit.