Well vent your anger out on here then. He’s one of your best mates for lots of reasons, don’t fall out over one incident. I hadn’t quite got the festival thing and that you had paid for the tickets, yeah I can understand your frustration and anger. But it might be more than just a case of them not thinking it’s important, it could be coz him and the missus are having problems, he might be a bit short of cash and need to work the week-end, he might need to visit someone for whatever reason that weekend.
It could be a million and one reasons about which he hasn’t, and perhaps doesn’t want to tell you about, is embarrassed about, whatever. And let’s face it, it probably is, as he had already committed to it.
Just let it go. Falling out with him won’t make the whole occasion suddenly become a reality, so it’s pointless. Try and make the best of it or organise a last minute alternative.