I can't believe that so many people are making this your fault. She's an adult
She said it was in her diary. She said she was looking forward to it. Therefore, when she was considering going on holiday she would have seen she was already booked and contacted you to check.
On top of that - realising that she's messed up and you've paid for something that she now won't be about for - she hasn't had the decency to ask if she can reimburse you.
Sod that, I'd dump her.

I'd struggle to call her a friend after this to be honest.

If you can't get the money back from the weekend you have booked then see if another family member or a proper friend would like to go with you.

Step right back from the friendship with this person and let it cool.

She manages to remember the plans she flaked out on you in favour of. So. She can get in the bin.

That is the very definition of ‘setting her up to fail’!

You know she’s an absolute flake-out, you know she doesn’t put things in diaries, you know she forgets arrangements.

So - you made an arrangement months ago, didn’t mention it again, and assumed she’d magically go against everything she’s ever done before?

Why……..?

She’s behaved appallingly, but she was always going to behave appallingly. Now you decide if the friendship is worth it.

Crumbs! That's a lot to spend on a friend (even if half was for you!)

I agree with @OohMrBingley in last 2 posts.
You start the thread saying that you know what she is like, then set her up to fail.
That isn't the way I'd treat a friend. If I had a friend that I knew struggled with organisation and often didn't remember things, I'd have been sending them weekly reminders of anything I were booking for them - because presumably, despite her flaws, this must be someone you consider to be a good friend if you are booking all these things for her birthday.

So you have strategies.
She is an adult who should care about her friends.
She should have strategies

How does she function at work?

Does she have a 'mum' there that constantly reminds her of what to do?

Vote with your feet and don’t organise anything for her again.

I think you’ve been so generous with her that she takes is for granted.

How annoying.
I too have a friend like this. Inviting her once for a meal at mine while DH working away. Got all the food in, organised childcare and cooked it all. She didn’t arrive. I telephoned her and she had just had her dinner !! She had forgot. We only discussed it three days before. !!
P.S. I need a friend like you and I am always up for a spa 😂

Eh? Which part of organising a weekend away is intense??

Your friend was 100% wrong to have not put the weekend in the diary and checked before booking other plans, but I also think you're 100% wrong not to make any kind of follow up until a week before your plans.

I wonder how close you actually are if she has never mentioned she was off on holiday to you before your message? If I was going on holiday, especially for a special occasion - like a big birthday - or as a gift from a friend/partner, then I'd be discussing it with my friends as part of the general discussion of what's coming up in my life, but also wanting to discuss things to do/where to go/asking advice if I know they've visited the same place previously etc. Also at only a weeks notice it's not like she can cancel the holiday, but if she had had more notice she may have not booked it for that weekend or been able to let you know that it was booked so you could switch to a different weekend, especially as she had no idea what you had booked.

You know she is a flake and you know that you decided to do "something" this specific weekend but made no further concrete plans since the summer. Why, knowing this, would you not mention something at some point before now?

If I had arranged a weekend with a friend for a birthday, as soon as I booked something I would be letting them know to confirm the date still worked. Even if it was a "So excited for your birthday weekend, I'm not telling you what I've planned as I want it to be a surprise but just checking that Saturday 6th is still good for you because I have lots of plans for the whole day!" ... Followed up with "Right, everything's booked for Saturday 6th, I'll be picking you up at 9.30am for a weekend of birthday fun!" especially as she might have assumed your plans were "fancy dinner on Saturday night" as opposed to "a full day of activities all day Saturday" and it would have given her time to adjust her plans accordingly.

You sound like a lovely friend OP. I would love someone in my life to surprise me with this level of effort. I hope that someone appreciates this and offers you the same investment of time and energy!

This

Do you know what- once I totally forgot I was meeting with a good friend who had booked brunch for us. It was very rare kid free time. I had 3 very young DC at the time and was out somewhere with them. I had only been speaking about it with her the day before.
She turned up to pick me up and I was not there.
To say I was mortified was an understatement. I don’t know what happened but it just went totally out of my head that morning and I went onto auto pilot doing my usual routine. I made it back when she called and we did go out just a little later.
I value the friendship so so much. I apologised profusely but felt my friend was really disappointed in me underneath and no matter what I said she still felt let down. It happened years ago now but still makes me sad.

I've also had scenarios where - as I'm usually the organiser- people ask me to reconfirm information that is at their fingertips, should they make the effort to scroll up a couple of pages.

I've also double booked myself for book group because although we verbally agreed the date, it wasn't written down.

I just find it odd that when booking and organising all these lovely things, you never thought to say to scatty friend that you communicate regularly with " Dfriend just wanted to remind you that we agreed X date for your birthday day." until a week before the event.

I'm not saying it's your fault OP - it just feels that it's down to friend being scatty and a lack of checking the date with her between then and now.I'm glad you're able to get refunds for everything and go to the spa with your DSis.

@Lavender14 with kindness, this maybe how your neurodiversity manifests. Mine is the absolute opposite (ASD, dyspraxia, ADHD, OCD, PDA - ASD is dominant).
Although highly disorganised in my life generally, I am absolutely fixed on dates. If an arrangement is made, that’s it. I hate flakiness in anyone as it’s so unpredictable. I can only deal in certainties. But, again, that’s how my ND manifests, I know others for whom the experience is different.
However, many neurotypical people are just flakey. Personally, I cannot cope with the uncertainty of that have a history of withdrawing from flakey friends because I can only deal with absolutes. I’ve tried really hard to understand that we aren’t all the same and although last minute cancellations are still upsetting, I take it less personally. Rudeness and flakiness however is a whole different story.
Thats a very roundabout way of saying that I don’t think you can conflate flakiness with neurodiversity.
Flakiness is flakiness regardless, and many of us find that very difficult and upsetting to deal with.

This sounds a bit of a lop-sided friendship.
Did she really forget - or did she just get a better offer?
I'd be taking a big step back and see how she reacts to that. Does she text you? Suggest dates to go out? Or does it just dwindle away to nothing?

I had a friend like this. I don’t any more. I’m not sure I could get past this.

Absolutely this. And OP has no answer.

The date was arranged way back in the Summer. Then you did a lot of arranging of stuff without her knowledge.

And never mentioned it again to her until the date was nearly here.

If she's a good friend how come it was never discussed?
How come you didn't know her travel plans?

If one of my friends did that I'd think it a bit odd that she'd done all that and never mentioned the birthday treat day any time we spoke.

You were thinking a lot about this friend and planning it all out in your head.

People have busy lives. It doesn't mean your friend doesn't love you, maybe she just didn't think it was such a big deal, or that you'd forgotten because it was never mentioned for the past months.