Oh pudcat. You must be feeling so sad. Has your mum only just recently moved there?
It sounds like a great nursing home to have organised such a lovely afternoon and to have invited family, too.
I can't help at all, but, I'm sure she'll have a nice day, and though it's no consolation to you, if she's unaware that it's Christmas, she won't be expecting a visit and it will come as a nice surprise when you see her on boxing day.
My DH's two grandparents aren't local. One is in a home that is just awful (we would like her to move, but it's not our decision,) and the other really should be in a home but lives on her own, again it's not our decision or right to change the situation.
It's likely that they'll both have a pretty miserable Christmas tbh, which is just a wretched thought.
I can tell it's going to be tough and tinged with sadness, but try and enjoy your Christmas, and rest assured that your mum will have a lovely day with people who care about her. Do you have young DCs, OP? They're great for taking your mind off things!