What approach to take?

I think it would be simply ghastly to stay at that time of year with a strange atmosphere hanging about.

I would thank her very much for the invitation and say you just wanted to kindly ask if all was well and had you caused offence.

I would stay nearby, simply because I couldn't stand to be manipulated: pushed away one minute, under their roof the next.
Neutral ground means you can come and go easier.

The last sentence of your second post jarred with me.
Do you think that the gift of money for work on their house and your expectations from the gift could be the problem?
Not that I think you should stay in a B&B or similar as some posters suggested but you did sound as if you thought you were entitled to stay in their home because your gift money had been spent on their property.
This is how it came across to me and if you have shown any of this attitude to her, I don't think that it would go down very well.
You must realise that your Dil holds all the aces, so to speak.
They might be your adored Dgc but they are her children and he may be your Ds but he is her husband.
As you say he is trying desperately to pacify both you and her but you must realise that if there ever became a choice he would choose her.
I would tread very carefully!

Oh dear.

I am sorry if I appear critical of DS here, but he is actually perpetuating the problem by refusing to be open with you.

He knows why his wife is unhappy, and knows that YOU know she is unhappy, but appears to push you together when this is clearly not working.

Without being mean, this sounds like a recipe for a rubbish Christmas for everyone.

DS should have the kindness to everyone involved to be honest about what the problem is. Acting like nothing is wrong is only going to make things worse.

Would it not be better to stay in a rented place than in the home of someone who clearly - for reasons DS refuses to disclose - does not want you there? The granddaughters will pick up that mummy is unhappy about your visit.

Was the money for the improvements given under the condition that you would stay regularly in the home, or was this something that only came to light later? What expectations did you have when you discussed the gift of the money with DS and was DIL aware of this?

I do not wish to project at all here, but my DH has accepted gifts of money from my MIL that came with particular expectations, not told me about either the source of the money or her expectations, and it has never failed to cause trouble all round.

I think before you embark on a potentially very difficult visit, DS owes you an explanation of what the issue is so that you can consider how to solve it.

Yes but what is she to forgive ? and who is to forgive ?

smileless Perhaps you don’t believe in burying the hatchet, or least said, soonest mended.
Personally I don’t like confrontation and ‘having things out’ - I think that far from ‘clearing the air’, it often simply makes things worse.
I prefer to forgive and forget.
But we are all different, I suppose.

I shouldn't think it would be as easy as you assume janeainsworth when someone's refused to talk to you for 9 months.

Ignoring the 'elephant in the room' is never easy especially a the OP's son knows what the problem is and refuses to discuss it with his parents, therefore denying them the opportunity to try and sort things out.

Like you, I'd be unhappy if that had ever happened to me. That said I wouldn't have been surprised if I'd refused to speak to her for 9 months and made my reluctance for her to stay obvious.

Easy to act pleasantly , IF you know what triggers to avoid , Your son needs to clear the air

Gosh. If I had invited my MiL to stay and her response was ‘no thanks, I’d prefer to stay in a hotel’ I would have been really hurt.
Do you really want to make things worse?
How hard can it be to just act pleasantly for four days?

I agree with annsixty it could be awful if it were to happen again. Who extended the invitation? Did it just come from your son? If your d.i.l. has made her reluctance obvious I would need to get to the bottom of why she'd not spoken to me for 9 months before accepting.

Walking on egg shells for the duration of your visit would be extremely stressful IMO and walking into a possible mine field, even worse.

Stay in a hotel. It obviously makes your daughter in law unhappy and anxious if you stay with them, so make it easier. You can all breathe a sigh of relief as to go back to the hotel each evening, and you will see your grandchildren during the day. I don’t see what the money for improvements has to do with it.

If you don't know what IT is, how will you be able to know If you are doing IT again?
I would need the air clearing comprehensively before I visited.
If you are flying over, you will not be able to just get in the car and drive home if things blow up.

I agree Maggie.
It shouldn’t be a battle but behaving beautifully always wins.
Give her no weapons.

I agree with chaos try and keep things light, you may be able to find the cause of the problem once channels are open. Your gift of money for their house was just that - a gift.
Attaching strings is not going to help. I hope things improve for you all.

The killing with kindness strategies suggested by buffy make sense and have certainly worked for me in the past in a number of scenarios!

I think ChaosIncorporated has made an excellent suggestion and personally I'd go with that.
Your last sentence is a bit odd, so what if you contributed massively to their home improvements, you'll be able to see them and admire them over the course of the 4 days. You don't need to live in to do that. I thought the idea was to build bridges not inspect where your money has gone.
Offer to stay elsewhere it will certainly make things go more smoothly.

Yes Maggie, spot on.

Thank you for the interesting answers so far : yes, we think our DS knows what is wrong but he is torn between his wife and his parents here trying to keep the peace.
We fly from Geneva to Luton, so must arrive to coincide. Reluctance is from DIL's, not DS's side. We have given a substantial sum of money for improvements to their house, so do not feel as though we want to spend the festive season with someone unknown [Airbnb, B+B etc.]

I don't see how it can be made worse really. Would you still see your Grandchidlren through your son?

Have you asked your son what's wrong? Does he know?

Though in your case, crazyH, it seems you can’t do right for doing wrong!

I would email or text her to say you are delighted by the iinvitation, and want to fit with her plan, so....
when would be a good time of day to arrive?
to leave?
and, that you gathered from DS that the idea was to stay with them but wanted to doublecheck? if not, can she recommend a good local hotel.?

Make it friendly, light, and personal. Maybe ask what GCs might like for Christmas?
Then avoid anything contentious, and don't let alcohol take control of your mouth at any point (I have no idea whether you drink at all, or at what level, but alcohol is a massive factor in seasonal family disputes!)

Yes, it is playing it "her way" but the invitation is an olive branch. I would accept it as such, and try to move forward with a fresh start.

Some good advice there, Buffybee. Disarm her with kindness. After all, you catch more flies with honey than with vinegar.

Well evianers, at least you have been invited over. Your son obviously thinks a lot of you and wants you there for Christmas.
I have a disastrous relationship with one of my d.i.l and I have no support from my son. He says the nastiest things to me and I feel we are reaching the end of the line. I have just had a nasty text from her accusing me of not caring for the grandchildren just because I didn't ask how the little one was doing at her new school. I was told to keep away and so I didn't ask. Now it's been thrown back at me.
Anyway, I feel I want to move far away from everyone. All of us live in the same town and I just hate it. Lord, take me away from all this, please !! He never listens anyway?

Crossed posts, buffy. I agree with your approach.

They have invited you, just go for goodness’ sake.
If you decline or make a fuss about staying somewhere else, your DiL and DS will be quite justified in being upset!