I'd just raid. Yes, it sucks that they didn't wait for you and the regular raid night. But in the great scheme of things, this is not worth fighting over.
Maybe make a post on your forums outlining how disappointed you are, and ask them to wait next tier. Or talk to your guild leader in person. But attend the raid tonight, and do your best. After all, you presumably like these guys and enjoy raiding with them.
It's not worth destroying friendships over something relatively small like this.