Since my family don't celebrate Christmas, I've always celebrated it with my ex-boyfriend and his relatives. We did this the last five years and I've been invited over this year as well.
They cook a traditional swedish christmas dinner. Some of the really good dishes and some are bad (really disgusting)... It's really nice as well since everyone helps out making the food out of scratch, even the small 5 year old kids. Last year I was in charge of making the meatballs. Which I did with my ex's nephews and nieces.
So for starters they always have glögg which is mulled wine. They also have "dopp i grytan" which is where they take the leftover oils and fat from cooking the christmas ham, make a broth from it and then dip pieces of dark bread in it (this is really disgusting, I tried it once, and ever since, I make myself scarce whenever that is served). This is all before the actual dinner.
For the christmas dinner there's a huge spread of food. The centerpiece is a mustardglazed ham, there's potatoes, a potato-gratin with onions and anchovy (called Jansson's Frestelse, I avoid this), smoked salmon, an assortment of pickled herring (the one in mustard is really good), They also serve home-made meatballs and prince sausage (small sausages) and the most disgusting thing of all... Lutefisk (white fish that has been preserved in lye over a period of time)...
There's also a lot of side dishes with cabbages and home-made bread with a lot of cheeses.