Just a few things that popped out at me:
My SIL booked a place for my FILs birthday that is coming up soon. She spoke to my DH about it who failed to mention it to me until nearer the time.
That's not her fault
we had plans already made and can't change them yet the meal can easily be moved to the next day as I've checked with the place already.
So she arranged something and you went behind her back...
I don't think this is my DH fault as he is a lorry driver who is at work alot!
I get working long hours, but I don't see how this makes a difference. He still has the ability to double check things.
As he didn't want anything to do with it as he was too busy!...
Again, that's not her fault.
And Sunday does suit everyone
Are you 100% sure that the others haven't made other arrangements on the free day as the event has now been arranged for a specific date? Generally I have found that things get arranged for Saturday rather than the Sunday due to work the next day/it might be seen as more of a relaxation day. Do you know what time everything is supposed to start?