AIBU to do DS's birthday on the Wrong Day

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Right, have names changed for this as need to include a few relevant details that would leave me Out Like A Brussel Sprout.

DS' birthday is Christmas Day, this year he will be two. Last year we did Christmas in the morning, then birthday cake and more presents (not that a 1 year old really cares) in the afternoon. I missed quite a lot of this as I was ill with raging tonsillitis and pretty much laid on the sofa trying not to cry while my mum and dad looked after us all.

This year we are going back to stay with inlaws for a week or so over Christmas. On Christmas Day we will be going to my parents (they live 1 mile away from inlaws, very handy!) for Christmas lunch. My FIL has to work all day on the day, except for a 2 hour break for lunch. They are coming to my parents for lunch with us. If we do birthday celebrations in the afternoon, FIL will miss it. FIL adores DS, he is the absolute apple of his eye, and DS adores FIL too. He wont be home till about 5 minutes before DS' bedtime and I don't fancy loading DS up with cake at 7pm.

I am proposing that we do DS' birthday celebrations on Boxing Day because:
a) both sets of grandparents are at home, so FIL will get to enjoy it all rather than having to rush it all in before bedtime. They didn't get to see him on his first birthday, so I think it will be nice if we can have a proper day of birthdayness.
b) No one wants to eat birthday cake after a massive Christmas lunch and half a tin of quality streets
c) DS is just going to get overwhelmed with presents and not care beyond the ripping off of the shiny paper.
d) He's two. He doesn't know when his birthday is.

Now, I think this is quite reasonable. MIL has made noises that it's not his actual birthday, so not fair that he has to miss out because of when it is. I accept this, but he's too young to know or care, and when he is older we will make sure we celebrate his birthday on the day too. I think they will go along with whatever we decide, and they are very lovely and generally not unreasonable people. So I'm just checking, AIBU? OR is this a very sensible plan, that will spread out the fun, help us avoid the boxing day post-Christmas lull and mean maximum opportunities for cake?

My dad agreed with his mum when he was about 7 that they'd move his birthday from Dec 25th to June 25th so he gets two lots of presents and a special day. He's 63 now and we still celebrate in June!

Last year I asked mumsnet whether IWBU to move my 4yo birthday by one day so her Daddy could be there (and she didn't even know) and got absolutely slated! I did it anyway, and she had a great time (and still doesn't know! ) YANBU and tbh, I'd be tempted to do it every year. After all the Queen does!

Move it so you can celebrate separately

We did DS's 1st birthday a day early this year as his birthday fell on a nursery day whereas the previous day was a Sunday so everyone was around. DD was 4 thus year and knew when her birthday was as we'd been counting down for months but, again, it was a nursery day so she just had a card & present from us that morning & a mini roll or something similar for her post nursery snack rather than the usual banana and we did the rest at the weekend.

Ds will be 4 on the 12th dec. I will be 40 weeks pregnant on the 2nd. There is no way I'm doing a children's party at that stage/with a newborn so we have brought his birthday forward a month (as all other weeekends booked up) he has no idea that he won't be 4 on sat. Even his nursery have agreed to do his birthday celebration early. I don't feel bad. He has no idea. It was either this or didn't he have a party and as this is the first year he's really been aware its his birthday and that people can have parties then I didn't want to do that.
Yanbu

Thanks for all the replies and wow we've got a lot of festive babies! Must be something about Spring...

Glad to see IANBU, I think we'll go ahead with a Boxing Day celebration for now, and when he is old enough to be aware of having parties etc we'll ask him whether he wants to do something in January, or even in the summer instead. I'm too lazy to throw a birthday party for him when he's this little, but think we might have all his little friends (aka my friends with similar aged babies) round after Christmas for some cake and a playdate.

YANBU