To expect work colleagues to go for meal on theirs days off?

I can beat this level of entitlement.

Years ago when I was fresh out of university, I worked minimum wage at an office. The CEO’s PA announced via email that there will be an annual teams dinner to celebrate the progress made over the previous 12 months. This dinner would be held in the evening outside of work hours, and we had to pay for the set menu dinner upfront ourselves (public sector company, but still). She also left details of how to make payment. I replied explaining I wasn’t sure if I could make it (in reality I could not afford it) but I’d let her know if I can as soon as possible.

Shortly afterwards it was my birthday, and my mum
posted me a big warm winter coat from oversees as a present. I posted a photo of myself on a walk in this big coat and captioned it “new jacket”. I had CEO’s PA and other colleagues on FB at the time, and PA comment “So you can afford a new jacket but you can’t pay for your work’s meal 🙂”

I had stern words with her the following day and she never spoke to me again.

I wouldn’t go on a Saturday unless I was really good friends with a colleague. I am only good enough friends with 2 people. Anyone else I’d go after work. If is difficult to become friendly when you wfh.

Hmmm the subtlety has evaded you
There are only 5 in the group as per the Op
If they decline she wont have her birthday celebration
Its subtle pressure

Honestly, you're not wrong at all, asking WFH colleagues who aren't even friends to give up a precious Saturday evening for a birthday dinner is a huge ask. Weekends are sacred, especially when you work full-time, and mixing work boundaries with personal weekend time for people you don't talk to outside of direct work tasks feels pretty tone-deaf. The easiest way out without causing unnecessary drama is to polite-decline with a soft, undeniable excuse. Just tell her something like, "Happy early 50th! Unfortunately, I already have weekend plans that I can't reschedule, but I hope you have a wonderful time celebrating." If you want to keep the peace and be nice, you could offer to buy her a coffee or a quick drink during the workweek.

Actually, I agree with most here that I would not want to go. BUT there are so many posts on MN about how difficult it is to make friends after 40-50 that I can’t help wondering if in fact she doesn’t have anyone else to celebrate with. Which isn’t your problem obviously, but it is quite sad.

Is it really that "bizzare" to not want to spend my weekends with people I don't really know? Ok then.

I had the same with a colleague’s 40th, also on a Saturday and in her house. I think she was quite lonely and was the annoying office attention-seeker. I didn’t go. I said I had family commitments at weekends.

Given you don't all know each other well, I'd find this odd. I'd imagine no one probably wants to go and is waiting for the first excuse so they can piggy back off it. Issue is if you all say busy "x weekend" she might suggest another.
I'd say I have other commitments at the weekend unfortunately but what about lunch at work (or something, if you want to that is)

You have no idea what her expectations are, or if she even has any.

Perhaps a party on Zoom is the answer!

MN is generally a place full of anti-social types who generally seem miserable and depressed. I’d be happy to go if I didn’t have any other particular plans.

Its actually full of people who comment without reading the full Op or understanding the context

I’ve learnt with things like this to day no straight away with no reason and try not to think about it again.

It was based on Frankie Howard's 3 nays....

I can hear him saying that now

They all wfh, so a drinks meet up would involve more travel than simply hitting the pub closest to the office.

Why not? It's nice for work teams to have a bit of social time. And what's wrong with a Sunday?

Personally, I would not go. I understand why you feel obligated because there are only a few of you, but really, you do not have to go.
Neither do you need to go into lengthy reasons why you don’t want to go, just say that unfortunately that time does not work for you, and leave it at that.

This is a bit strange. In her post the OP says they all work from home, and yet when posters suggest that going out after work would be better then OP agrees? But surely going out after work would be just as ‘weird’ and inconvenient (if you’re going to see it that way) as going at the weekend.

So many posters are ignoring the work from home element. It’s pretty obvious to me that if they were all in an office the birthday lady would be suggesting after work drinks.

This all does rather raise the question of whether anyone can ever again make personal connections with colleagues when working from home. There’s such a miserable attitude of ‘I don’t know you and therefore I don’t want to get to know you. We’re cut off from each other during the week but the weekend would be weird and anyway weekends are precious’. I mean fine, but I think we’re worse off for it. I feel sorry for the lady.