No, you're not being awful. I'm thinking it's tricky because you told her the name of the church? So she could just show up since it's open to whoever? You clearly told her you and a friend of yours had made plans for Christmas. Just say that... you've made plans with someone else. You don't need to explain what those plans are. If she presses for details, just say I'm leaving that up to my friend, they just told me when to be at their house. And if you decide to stay home, you don't have to tell her that or answer your phone or door!