To answer a couple of points, a brothers weekend was organised for a specific event for them to celebrate DH's 40th together. BIL the. Went and accepted an invite to a wedding abroad and another brother had to practically pin him to the wall to get the truth out of him. The event can't be moved, and he'd already signed up for it before accepting the wedding invitation. So DH will be celebrating without that brother.
So, unmovable event, significant birthday, was agreed first and then he avoided telling us he now couldn't make it.
As it happens we should be thousands of miles away on the date he's chosen for a once in a lifetime event that's been planned for years. It's also an immovable date. We could cut the trip short and arrive home just in time for the wedding, but I don't think either of us are minded to.
We don't celebrate our anniversary, but one of the other brothers mentioned to DH that it was odd the other brother had gone for the same date as us - none of the made a weekend of it when we got we'd, so not very likely to this time around.
And as I say, we've already planned to be elsewhere.
I don't care about the wedding at all to be honest, so my perspective on DH's upset is probably skewed.
Thanks for all the opinions.