IL's not thanking my parents for hosting Xmas day

[smartiecake]

Every year for the past 10 years I have hosted Xmas and cooked for 12+ people, my parents, sister and her family and my in-laws (DH is an only child) so that we are at home and everyone sees the kids and this has been fine. This year my parents hosted, cooked, bought most of the food, lots of effort. Again all of us together and invited my inlaws along. Had a good day and were there for about 9 hours. Today I find out my FIL (v quiet) left without saying thanks or goodbye to my parents, although I am sure MIL did. I am really annoyed by this. What do I do? I want to raise it with the in-laws and say I think it was rude but it could cause a falling out with them and upset DH.
DH doesn't know yet but I will tell him. He is unlikely to want to say anything for fear of rocking the boat but my immediate reaction is they can be on their own next bloody year then but then I am mean.
What do I do? I am fuming

...if not MIL would have been in a right huff about it and said so.

And that^ would have been extremely ill mannered of your MIL!

MIL would have no hesitation in speaking her mind

How was his behaviour throughout the day? Was he pleasant and chatty or just silent and miserable?

He exchanged pleasantries when we got there but he was fairly silent and miserable all day.

Maybe he is shy. Maybe he was never taught by his parents the correct thing to do. Last thing you need is causing trouble in your own relationship because the old grump went off. Leave him off. Everyone doesn't always know the correct thing to do.

The difficulty is if we don't see them they will be on their own all day - no other family only DH and us, they are young, late 60's recently retired and bored with little hobbies. I know I am not responsible for them but I make an effort to make sure we do stuff with them and they are included. I don't like to think of them being on their own as that's the same as every other day for them, but they are hard work sometimes

There's nothing you can do but assuming no Sen how can an adult get to the age of and 60 and walk away from hosts after a day being entertained by them without a word of thanks??

I also have a silent fil and it annoys me that he is allowed to get away with zero social effort, sits there in silence with a long face. I got bored of keeping it all going so mirrored him and was quiet also. Within minutes both he and mil were asking why I was so quiet. Annoying!

Maybe he's an introvert. My df is very quiet and would also slope out leaving mum to talk to and thank the hosts. What does it matter if thanks were given?