I would suggest bringing her home for lunch, and then telling her you are going out for tea, and getting back to the CH for teatime. All this assuming you live close enough, of course.
Having said that, how long has she been in the care home? Will she be aware that it's Christmas?
We brought my FIL home for his first Christmas after he went into the CH but it was a bad mistake. He was confused and/or agitated for much of the time, and we wished we'd listened to the CH staff, who'd said he'd be better left in his safe, familiar routine. But I should add that I think we'd have felt bad if we hadn't tried.
We never repeated the experience, though, and when it came to my mother we would just visit on Christmas morning. To be honest, after she went into the CH (and even for a couple of years previously) she never had a clue that it was Christmas, even with turkey and crackers on the table, and people giving her presents, etc.