I'm going to pretend to enjoy Christmas, then leave after all the gifts are open.
Why would you leave the party after all gifts are open?
I feel like it is a good time to leave.
Rather than pretend, you should try to have a good time at Christmas. You should enjoy the time with family and not leave after the gifts opened because it would be rude.
I know everyone is going to tell me to have a great time and Christmas is the last time we can be with family because of the holidays, but I get so bored of being around every single person in my family even for one day.
Just think of it as something that everyone else is probably thinking too, so just be there  the entire time and just go with the flow. Take your phone out and play games if you will to keep from being bored. At least you would have showed up.