How to handle an odd situation

There are a lot of possibilities here. I think the generic, "What are your plans for Christmas?" could work well. It might be awkward for you if the answer given still doesn't give you closure though.

It could be that your son is still working out arrangements with the ILs and will speak to you about plans when that is organised.

It could be they are doing a short visit for some reason and don't have a full day to spare.

It's also possible that something is going on in DIL's family and they are travelling for that. A couple of years ago I travelled around Christmas time due to a medical issue in one of my parents. I flew in. They told me not to hire a car as they were happy to take us to and from the airport. Each day I travelled with them to the hospital. I had no transport of my own. If there were any family members or friends in the area I'd normally have caught up with, I had no time of my own to visit them. I'd also never have asked my parents to have to work out fitting in any additional plans of my own under the circumstances, even if other people were willing to come to me. They had enough going on. The object of the trip was single purpose. I hope everyone is well on your DIL's side, but just an idea.

A dilemma indeed! Presumably, you chat on the 'phone with your son? I think I would avoid mentioning your granddaughter’s comment but just ask casually what their plans are for Christmas. You could perhaps introduce the topic by talking about your own festive preparations (shopping, baking, decorations, cards, etc) as a lead into the topic.

It could be that things aren’t finalised with other members of the family, who may well have work commitments. Are you retired? Is it possible that your son, rightly or wrongly, assumes you’re available, even at late notice?

Good luck and I’m sure you’ll be diplomatic in your approach. Perhaps you’ll keep us posted on the outcome.

It's really difficult, but personally I couldn't let this lie - but that's the way I am. I would try to bring it up in conversation, you know along the lines of "what are you doing this Christmas holidays? Anything exciting?"

They may be planning a surprise visit to see all the family, but then the 'What ifs' niggle at the back of the brain as to how to prepare for the hurt if they don't come to you. Or they may be hoping to come, but haven't said anything yet as Covid may still rear it's head and put a ban on travel again.