Thank you for so many replies, I didn't expect that-and for so many different perspectives.
I'll try to answer as many as I can.
Perhaps crucially, DP doesn't work on a Friday but works Thursday night (until 0630 am ish) Usual routine is they go the partner/birthday person's house after work, sleeps while b'day person works and wakes up the same time as they finish. So they get Fri night, Sat day and night together.
The DP absolutely will not hang about on a Sunday. They state they have to be ready and in the right frame of mind for work, not rushing about at all. There may be a drop of SN (autism?) applicable to that if that matters. They're upset as they were looking forward to it.
Point taken about the host's use of 'Ass out of U and Me' comment-they also assumed the birthday person knew-unless a PP is right and this was somewhat contrived.
The Sunday thing is very common knowledge to host and others involved.
Taking the point about how much effort other LDRs haev had put into them for PPs who've been in those.
I never thought that they might be another 'thing' going on with them.
Host does go 'all out' lots of food and drink, huge marquee-they have a beautiful large garden, quite a 'naice' BBQ.
I am sorry for being confusing, perhaps I should have used names.
Birthday person is largely irritated that the host discussed which day would be best with another family member they're very close to (host that is) but didn't discuss this with them at all, and the BBQ is apparently (50%) for them.
I've answered this before but there was no personal reason for it being on a Sunday, host just had a conversation with another attendee and they agreed on the Sunday but didn't consult the birthday person about the conversation or the decided day-had they done that, birthday person would have spoken to their DP and ascertained if any flexibility could be awarded and if not said they couldn't do that day. May have been the same outcome, may have given time for the DP to take AL, may have resulted in them changing th date. Definitely wouldn't haev meant the two involved parties would have been looking forward to the event then to suddenly find out it wasn't able to happen.
Host says they said the date but DP/birthday member don't recall this at all.
I don't think the Sunday is for fear of drunk people-most guests are quite elderly and/or don't drink. The DP will be driving, and no trains from around there anyway.
The DP doesn't mind doing the cooking at all, that's not an issue, and host always appreciates this because it frees them to talk to people rather than be stuck behind a BBQ all day, so isn't happy that it'll no longer be happening. They also are always helpful tidying up after etc. and usually one of the last to leave. It's not a late thing, starts in afternoon finishes evening.
Just thought as well. If friend hadn't messaged at all, the Bday person and their DP could have just rocked up on the Saturday with their contributions and wondered where everyone was (which is what ALMOST happened with the aforementioned neighbour's gathering)!
@Squeaky2023 I think that's how the host feels but it didn't occur to them that the Sunday might not be suitable. I also think birthday person feels-they're wishing their DP would make an exception for what will be a nice 'do'.
@Yellowflower47 yes in their forties.
It is perhaps possible @PriOn1 that the host feels the birthday person is not realising that their DP 'just isn't that into them' but I personally feel this is a pretty crap way of 'proving' that after both birthdayperson and their DP had been talking about it with them and looking forward to it.
I did wonder about how reasonable it is to assume things like this BBQ would be on a Saturday-host says not to assume but host assumed the birthday person knew it wasn't.
*It sounds like they’re caught between a rock and a hard place with a host that’s been careless and a DP who isn’t that committed This has made me quite sad to think about *
@Codlingmoths IT's odd because host was aware that this person always leaves on a Sunday early, but may have thought they might not this time.
@Marsyas that is a point but, the birthday person has often been upset at the DP never sticking around on Sundays and has confided in host, so they're very aware of that situation, rather than it being something they could easily forget about.
@Beautiful3 that sounds infuriating!
@Saschka birthday person does kind of 'half know' some other people there, but they all know each other well? If that makes sense. The closest age gap to them with others there is twenty years. Bit of 'spare part syndrome' felt without their DP.
@AlisonDonut 
sorry!
@EliflurtleTripanInfinite yes, I'd do that too but I am a bit of a 'helicopter host' to be honest-I fuss a bit too much whenever I haev people over or so I'm told!
@knockyknees that's what the birthday person feels should have happened-they should have been privy to the plans before they were set
@whatkatydid2013 the host is feeling put out definitely because of the birthdayP being upset and saying they may not be there all day, and that they won't receive help-the DP is a better cook (has done it for a living before) calmer and generally makes things a lot easier for the host.
@Codlingmoths I'd have done that too, just worked around it, been a bit tired etc. Not a big deal but the DP won't.
@Twillow I didn't want any bias that's all. Host didn't know that people preferred Sunday, they just decided others might be busy on Sats throughout summer.
@pikkumyy77 yes, bday person will still attend, but they've said they may not be there all day given it isn't what they expected and they're not really close to anyone there bar host. Never an option to not go at all.
@Begonne no, definitely not. They'll suck it up as others have said. they're just disappointed as were looking forward to it and now It's just something they'll go to and try to enjoy but miss their partner and feel it's unnecessary that this happened. They probably won't stay as long.
@saraclara I am like that too. If something's important I can 'suffer' a little bit for it.
@RobertsRadio no, it didn't occur to them to say about the day. No cognitive decline anywhere either.
Birthday member didn't 'rely' on this BBQ and has done their own gathering for their birthday. This is an annual occurrence and the birthday element is always perpetuated by the host.
@MrsDrSpencerReid I take that point. Again, it makes me feel quite sad.
@CarPour they do know people, just not very well other than the host, but all other people have others there that they know very well. I am not sure about other questions, sorry
I really appreciate everyone's perspectives. Aside from those who've misunderstood slightly (I understand why!) it seems It's more a case of different people feel differently about different things?