How would you arrange this??
Totally aware that AIBU to be thinking about Christmas this early 
This year DSS will be with us from 12pm Christmas Day, to 12pm on the 28th. This includes Christmas dinner. (He was meant to be with us all day last year but his mum's work shifts got changed, hence the half day (as a kid of divorced parents I hated half days, so I disagree with this, but whatever)).
Because we were meant to have him last year for dinner, I offered to do dinner for the ILs. This was actually really nice, apart from the downer of DSS not being there with us.
This year, my SIL wants to do dinner. I have massive food anxiety, and would feel seriously uncomfortable eating there. ILs are aware of the anxiety but there would be raised eyebrows if I didn't eat and enjoy the whole meal. She has been desperate to do this as her and her partner moved in together this year and they would love to host, which is totally fair.
MIL works in retail and cannot get Boxing Day off - otherwise I would suggest we do Christmas Day #2 at ours on Boxing Day and have all the ILs there then.
My mum absolutely loves Christmas, however because of last year's change of plans, we only went round late in the evening last year. This year she'd like us to be there for lunch and the afternoon, and would be overjoyed for DSS to be there too. She is very much of the opinion that we were with ILs last year so it is "her turn" this year. I'm inclined to agree, I really missed being with my family last year. Note - the journey to my mum's is an hour's round trip, so going for the morning only isn't really an option for us.
DH doesn't agree with me at all - he thinks that the most important thing is for DSS to spend the day with his blood relations (ILs), and we see my family either on Boxing Day or late in the evening again on Xmas day (not really workable as DSS goes to bed roughly 7pm).
My current thinking is:
Take DSS to SIL's 12-2.30 to open presents and have snacks. We see all DH's immediate family.
Decamp to my mum's to arrive for 3pm (this is when she serves dinner - hence DSS having snacks at SIL's. Stay till 6pm ish, then head home and put DSS to bed.
This way we spend roughly equal time with our families on Xmas day.
WIBU, me or DH?