Let your wife’s parents and others who will attend the dinner know that your Dad has dementia so they understand your situation. Suggest ways to make him feel comfortable, are there subjects or activities he might like to talk about, or listen to? Letting them know these things in advance will prepare them, and help them feel more comfortable also. Plan the visit, what time you arrive, what you will do before dinner, and stay with him until he feels comfortable or meets other he feels comfortable with. Make sure he knows where the bathroom is, and has assistance if he needs it. Plan for dinner to include foods he likes and give him assistance if he needs it. After dinner, he may want to stay or be ready to go home, honor his wishes either way. Christmas dinner is a very special time, and paying close attention to your Dad and helping him through will probably be a special time for you and your Dad.