Oh dear what's the solution here?

[Glassfulls]

Friend and I agreed on a weekend away. There was some tooing and froing about a date, we settled on two possibles, picked one then changed to the other. That was all agreed and we're going on, say 18th.

Friend has booked the hotel. She sent a message saying £X , 11th OK to book?. I didn't check the date because I knew we'd already agreed dates (and she's usually very organised, would know better than me) but it turns out she didn't change the date in her diary.

Both of us agree that it's been booked for the wrong date, there's no ambiguity regarding the date we settled on.

So she made a mistake, but she did give me an opportunity to check and I didn't.

I really can't go on the original date now, I will be out of the country with work.

I think I know what I'll do, but in case anyone has a better solution, WWYD?
.

Thy both made the mistake. Friend checked with OP, who presumably incorrectly confirmed she was available, when she's not. It's OP who is pulling out, friend is presumably happy and willing to go.

They both made a mistake, it’s right there in the OP. She double checked the dates with OP and OP said yep that’s right. They both got it wrong.

Pay half, as it’s both your fault, possibly even more so hers. Why can’t she go by herself? Or if she finds someone else to go you could give them a discount on your half, if it’s not the type of trip they’d usually choose.