DH's family always celebrated holidays on the actual days. My family, due to work on holiday schedules, blended families, and distance, often celebrated on days other than the actual holidays, like the weekends before or after.
My mother has lived with us for 25 years. (To the pp in post #61, sometimes it's just easier on us to help care for someone in our own homes than from afar.) As she was younger when she first came to live with us, she'd often go off for several days with her sister for holidays, or one of my siblings. But in later years, she couldn't - her sister became ill and passed, and mobility became an issue for her, and it became difficult to climb stairs and such, so she no longer accepted invitations to go away for a few days over the holidays.
I'd say for the past dozen years or so, at least, she has come to all holidays at my in-laws as an invited and cherished guest. On both sides of our families we are "the more the merrier" type people and often have whoever someone wants to invite and no one bats an eye, really. My mother had a favorite spot to sit at my FIL's house, often with a glass of wine or cup of coffee. I'd keep an eye on her, but watch as all day, various family members made their way over to sit and talk to her, some staying for quite a while and seemingly really enjoying the conversation. I think many of my BILs and SILs realize that, of all of us, there are only three elders left at this point on that side and value some of the things that a person of that age literally brings to the table. At any rate, she became part of the holiday norm.
As we fast approach the holiday season, I sit her a little weepy writing this post because my mother is currently on hospice in my home and I really don't know what the holidays are going to bring for me, and us, this season. I think back on all the many holidays over the years and how the people and locations evolved and changed, who was there, and how sad it was when beloved people were no longer there. Year to year you never know what will happen. I think on some level you have to cherish and appreciate each one for what it is, and who is there, even if it's not "perfect" the way we may envision it. Each person there has value to someone and in their own way. That's my best advice today, I've got nothing else.
BTW I really loved many of the posts here, including the OP's.
And OP, I got what you were saying about the dogs, too. (They can be put away when annoying! Ours often comes with us but remains crated except for the family mid-day walk.)
The reason it occurred to me in the first place is that I have a close relative that felt very similarly about her in-laws as you mention feeling about your MIL. Nothing earth-shatteringly terrible ever happened between them - it was just a clash of personalities and they never really got along. She did, however, recognize that her husband loved them dearly and felt very torn by the tension between them. She's no saint herself but for the good of her husband and her marriage, she grits her teeth and gets through things like family holidays. It's a wise approach and as tough as it sometimes is, it's easier than living with the consequences of excluding them and breaking her DH's heart.
