I am so upset, FIL has refused to go with our xmas plans this year and now I don't know what to do.

Either way he's going to have a different routine.

It's not your responsibility to ensure everyone has a wonderful Christmas. Like pp have said, you've invited him, he's said no. It's on him.

I mean he has every right to decline, your husband doesn't "have" to do anything

You're being unreasonable IMO

Is he meant to go to your mums when he doesn't want , to appease you? So that he doesn't have to spend Christmas alone, again, to appease you?

Yes bring him a lunch and then have him over to yours for mince pies and Christmas movies in the evening.

Why are you so so upset by his saying no. It was an invitation not a gulag order. One can say no to invitations. You have the kindness and grace to accept that things might be difficult for your ageing mum and have made adjustments for her. Yet you dont seem to have any empathy that FIL is also ageing and may want things differently or might feel overwhelmed by a change in settings. Dont take this personally.
You seem to be at the end of your tether with pleasing people so its likely less about change in your Christmas plans and more that you are thinly spread. Make a list of all things you are doing for people that are spreading you think an tick one or two off an stop doing that.
Oh and get used to things changing. This year, your DS is spending some time with his GF. Next year he might not be there at all. They might go to her family or to Spain. Who knows. And then DD will bring her "very different" boyfriend who might suggest that they do something different for Christmas.
Yes it hard changing things after 35 years but it is also an opportunity to do things differently!

This.

Tell him thats whats happening and he's welcome to change his mind and join you.

This is HIS choice.

You shouldn't be acting like doormats trying to constantly appease cankankerous old men.

So don't.

Simple.

It's an invite not a demand! He declined the invite which is his choice not yours.

Please do not do this (the guilt trip him nonsense at the end). He is entitled to spend Christmas as he chooses, why should he be made to feel bad about it? Ridiculous!

If he's not bothered about being with you all on Christmas day then why should your family spend it apart? You all go to your Mum's, DH included. FIL can come or not. If he was worried about being on his own, he would go with you.

This.

Just stop. Stop trying to control everything. You can't. So just stop. Honestly, you'll thank yourself.

Leave your FIL to it. Keeping on at him could be seen as bullying. Although it has been a tradition for many years maybe he has just tolerated it for an easy life till now.

He might know your family but I am guessing last year it reminded him of his lost wife.
He’s an adult - he knows the decisions he is making - unless your hubby disagrees - take him a Christmas meal.

I can't believe anyone would suggest that just because it's this old man's 5th Christmas without his beloved wife he won't be grieving. Time may help him to cope but it's especially difficult at Christmas & always will be.

This. With bells on

Maybe he really doesn't want to be surrounded by so many people. I'd be tempted to let him crack on with it but would definitely pop in to his at some stage for a glass of bubbly/cup of tea and exchange a few presents. But really it's up to your DH if he is happy to do this. Christmas is so stress filled at times and there are so many of these types of posts it's all really sad! I totally agree with you though OP; I think you're doing the best you can to support everyone and it sounds like a great compromise but you can't control or really influence how others may feel about it.

As a Christmas non traditionalist I always struggle to understand the madness that goes along with December 25th. I think you are upset that you always do things a certain way and that he is stopping you from doing that? You need to talk to your husband and decide whether he goes along with your nuclear fanily’s Christmas and visits his father later in the day OR prioritises his father and changes your usual routine. It’s really in his lap here.

Respect your FIL's wishes not to go, he may well have his own reasons for having a quiet day. Your DH joining him for breakfast sounds like a good idea though, and you can always let FIL know if he changes his mind he is welcome to come later. As you all live close by that's easily sorted. Just don't make a big drama out of it.

This is your FIL 5th Christmas without his wife - he's not freshly grieving as some have implied. It's our 6th Christmas without my mum; my dad has made plans that work for him for the past few years.

If FIL prefers to stay home for Christmas Day, that's OK, just respect that, @Dreamsfallapartattheseams . My Dad is the same - he wants to be in his own home or ours (which is almost his home) but very much does not like Christmas Day at anyone else's. He would far rather spend the day by himself pottering and listening to the radio than "being a guest.' He's an adult, that's his choice to make.

You're trying to impose your vision of Christmas - 'everyone together' - on someone who doesn't want that this year. You're doing it out of kindness and love, but you're still doing it.

Leave him be. Don't split up your family meal. Drop gift around on your way to your parents, perhaps, for a short visit if that's something FIL wants, perhaps.

I'm so sorry to hear about your mum's dementia. It's a very cruel disease. My heart goes out to you and your father.

Can you talk to him and explain your DM has dementia so she can't really go to yours. But you really want him to come, and that it's still you doing the lunch he's not imposing on your DM it's just a change of venue. He really might not understand why you are changing things around, he may be very comfortable going to yours but not your DMs. I would hope there is scope to persuade him, a lot of how happy it would make you all if he comes and it wouldn't be the same without him, etc, etc, even how his DW would want him to go.

If that doesn't work maybe a bit of a guilt trip that he's making everyone's Christmas difficult, the family can't leave him by himself, but if he won't come to you all the arrangements will have to change. Of course he may (probably will) say he's fine by himself and leave him alone, so last resort.

Go ahead with your plans, your father will be fine at home, he will still eat and probably very well! He wants to be at home and I understand that very well, it's not a slight on you. You do a 'Christmas' celebration with him at a different time and place to suit you and him and stop fretting about it.

Maybe FIL just wants a quiet Xmas - and why not? I hate having to watch endless present unwrapping by children etc - just don't get it!

I feel like you are being really considerate of your mum's needs and those of your children, but not at all the thoughts of your FIL. I'm getting the impression you don't like him and so don't really care.

However long your tradition has been running the time comes when it has to change. One of those times is aging parents with different needs.

You don't say why he turned down the invite so its hard to make suggestions but as PP say - possibilities include DH visiting him in the morning, asking him to come for a late Christmas tea and a couple of hours with the DC or suggesting he come on Boxing day instead.

It may be his fourth Christmas without your MiL but it is "another change" which could also be a factor. Ultimately if his preference is really for a quiet day at home then give him a call on the day and suggest Boxing day.

You do nothing. You extended the invitation and he has declined, as he is entitled to. Your DH does not have to go there to be with him and you don't need to tie yourself in knots trying to accommodate him. I'm sure FIL realised that by saying no, he will be spending the day on his own - perhaps that's what he wants/prefers.

You just leave FIL on his own on Christmas Day. You’ve invited him, he declined, that’s on him.