Might be worth replying and saying that you are upset they can't make it and that it is a shame they weren't able to let you know as soon as they had a change of plans so you could have invited someone else or seen if there was a different date when everybody was available.
but that you hope they have a good weekend, you'll have a drink for them and will look forward to seeing them soon.
So hopefully they will realise that they have been rude and not make the same mistake again but know that you are all still friends!
hope the rest of the weekend goes well.
Do you think they accidentally double booked - it's easily done if you are talking about dates a month or two in advance, you sort something out for June 15th, your husband sorts something out for the third weekend in june, you both have your own thing in your diary, one gets onto the calendar, the other person looks to put their thing on the calendar, sees writing on it from a distance so thinks great, oh has written down that weekend on calendar without checking what was actually written and then you only discover closer to the date.
Or we once had family party - my mum had organised, a house - cooling party to celebrate moving out of much loved house to a new one. DH's dad had said he was having a family party in june, just waiting to confirm dates. dh promises we will go, as we don't have anything else on as he has forgotten about my mum's party. And it's very important to my dh that I go as he rarely gets to see his dad, maybe once every two or three years. So I agree, with the provisio that it is not the same weekend as mum's party - which of course it is. We ended up going to our own parent's party, luckily it was pre kids so didn't have to make them choose or decide for them which they had to go to.
But I think that we all ended up feeling a bit aggrieved by it - me and my mum because my dh had already said we would to her party (which was very important to her and me) and he changed his mind when something else came up. dh becuase we so rarely got to see his dad or many of his family so he wanted us both to be there as it was important to him and thought that it was so important that there couldn't be anything that was as important that couldn't be changed or apologised for not going to. dh's dad because dh had already said that we would both go before we had been given the final date and so he felt I was rejecting his family (even though I'd already told dh that we had already committed to that weekend and he hadn't passed the message on when asked about available dates)
so I guess what I am trying to say is that it is easy for dates to get double booked without there being any malice behind it, everybody gets to be upset but hopefully you will have a fab weekend and they will miss being with you.