Middle, I had experience of this over many years. I found that being alone overnight from Christmas Eve until Christmas lunch was easier than having to say goodbye to the DC at lunch time and being alone all Christmas afternoon and evening.
On Christmas morning I would make my favourite breakfast and listen to the radio. See if there is a Podcast you could save up for that day. You could find an audio book to listen to. There may even be something interesting on the TV.
Some years I made each child a special decoration to go in his or her room - cardboard, glitter, shiny paper etc. One year I had a cardboard box for each child, put the presents inside and spent time decorating the outside of each box with pictures, cut-outs, ribbons etc. Anything to occupy myself, perhaps accompanied by Christmas carols.
If the weather was anything approaching reasonable I would go for a walk and say, 'Merry Christmas' to anyone who passed by. There were preparations for the Christmas meal as well, although not sure if you're making that this year.
When the children arrived I stuck on the happy face and avoided at all costs letting them see how difficult I had found it being apart from them. (Agree with ChileConCarne about not giving off sad and lonely vibes). It was all about them, admiring the presents they had already received, playing with them, seeing them open their presents from me etc.
It's hard every day after a split but it gets easier, however slowly. Christmas Day is just one day. Make plans for Boxing Day if you can - are there any events in your area? Sorry if suggestions are of no use but they worked for me. I even managed to see it from their father's point of view - they spent the majority of their time with me.