I would have a deep sinking sicky feeling in my stomach if this happened to me, and feel like I wanted to burst into tears, if something that I had been really looking forward to for a special occasion had been changed for the complete opposite.
I would know that I needed to pull myself together and try to enjoy it because everybody else would get pissy that I wasn't being appreciative of such a 'lovely thoughtful surprise' despite the fact that they were overlooking that it meant that they were getting what they wanted and that meant that on my special birthday, the one occasion I get to choose a couple of days for me, I get all excited about it but then it gets snatched away at the last minute and it's about turn to exactly what you didn't want.
I would ask dp explicitly why, when you had said you wanted a couples weekend, he thought it would be a good idea to do a family weekend. Might be that your parents or sibling wanted to be with you for your special birthday and pressured him. Might be because he couldn't change childcare or whatever - but I would get him to explain as clearly as he can why he thought that you wouldn't have asked him for what you wanted - at 39 years and lots of weeks, you are old enough to know what you want and to say what you want. Did he think you wanted a big family celebration and weekend of nose to the grindstone but didn't want to say? Ask him how, for future reference, you can communicate to him that you are telling him how you absolutely feel, what you really do want, and that it's not to be changed or 'improved' as has happened this time as it has meant that you're upset because your weekend has been ruined and he is upset because you're not happy with his plans THAT AREN'T YOUR PLANS.
To say that the earlier weekend away together was to make up for it is frankly insulting - how can the weekend away together as a surprise be your present when you don't even know that was the case until several weeks later? And it wasn't in the cottage you wanted.
I would be saying that it's nice to have a family gathering but that you'll be looking forward to celebrating your proper 40th birthday at the place that you wanted to celebrate it at some point soon in the future.
I would also have a massive conversation with both my parents and my sibling (separately rather than together so they can gang up on you) and say how hurt you were that they didn't know you better than that, that you were so looking forwards to your birthday weekend and now it's just going to be a family gathering which is nice enough but that you can see that you're going to end up doing all the work as usual and not have the relaxing, child-free weekend that you'd been so looking forwards to (and that your parents at least knew about as they knew that you'd asked them to do the child care and I'm assuming had said how much you were looking forwards to your plans....
Passive aggressive? Maybe. Probably. But not nearly as bad as changing the arrangements so significantly for your special weekend. Because even if you do go away again, it's never going to be quite the same - it's not the same weekend, you'll always know that what you wanted wasn't thought to be important.
Hope you manage to enjoy it somewhere somehow eventually though...
(and I get exactly what you mean - I had my 50th this year, thought I'd arranged lunch with dh, which then dmum and dsis insisted on coming to and as they spent so m uch time faffing about where to go when I'd already said where I wanted to go there wasn't time to go there so dh didn't come - found myself driving along with tears rolling down my cheeks and it still pisses me off now. I know I did a good thing for them and that's good but like you - it's the not being listened to that hurts).