During our 2 years of being separated but living in the same city, S alternated between our houses week by week. But holidays we didn't stick to the schedule. Mostly, I took S because my family makes a bigger deal of holidays and XH's relations with his family have never been as close as mine.
The first year we were separated, XH came over for most of the day on Christmas Day. I cooked, XH did some of his old duties, and my mom was there. Then he went off to do I don't know what! I stayed in town that year, so S15 (then 13) spent about half of his vacation with me and half with his dad.
Last year, XH came over Christmas morning and S14 unwrapped all his gifts with both of us and we had some brunch food. Then S14 and I flew out of town to have the full deal with my sis and her family in ATlanta.
Although never acknolwedged, OW was around during all this. I just don't know how and when he celebrated with her, it was kept out of my view.
I think everyone makes different arrangements here. I think barbiedoll's point is really important. For me, it was a big deal to have S open his gifts with both of us, and I think it was a big deal for him too.
Good luck. As I posted before this is way too hard, especially for those of us who didn't really get a vote in the matter.