In my family it's always been tradition to celebrate Christmas Eve with my mom's side of the family. Way back when I was really young the gathering would either be at our house, my grandparents' house or at my aunt and uncle's house depending on whose turn it was to host that year. Later on we narrowed it down to our house and my grandparents' house, and now we just have it at our house so that my grandparents don't have to worry about getting their house ready for an influx of people or driving half an hour to get to the party.
Christmas Day we spend with my dad's side of the family, which is a larger gathering because Dad has more siblings than Mom does. Since my dad's family was so big we drew names earlier in the month to figure out who was going to get a gift for whom. There were separate drawings for the kids and the adults. When my grandpa was alive, one of my aunts and her husband always got him a mechanized animal of some sort as a gift. One year it was a cow that would walk a few steps, then moo, and another year they got him a pig. That tradition stopped the year my grandpa died, although my parents did choose an interesting way to end it - they got Grandma a gorilla that sang "Macarena." As the grandkids have gotten older the gift exchange has been modified. Now the "unmarried" grandkids do the name drawing while the "married" adults do their grabbag/white elephant gift swap.
Another tradition that has become a favorite of mine is the opening of gifts on the 23rd of December. We started that tradition a couple years ago so that we wouldn't have to worry about opening presents from each other while trying to get the house ready for company at the same time. It's pretty much always the same: my family goes out for dinner somewhere (usually in town), then we drive around looking at Christmas lights before coming home and opening gifts.
-Kim