Really, you're right. Whether we believe "month" anniversaries are silly or fantastic, is not the point. Whether you can go to the movies another night is not the point. Whether you could dig in your piggy bank and fund the evening out instead of your fiance is not the point. Whether we think the whole night out and 9 month celebration was even important is not the point.
He made a promise that he knew meant a lot to you. Then he got an invitation from a friend and accepted, knowing that would mean he would be unable to keep the previously made promise to you. The reason this is unacceptable behavior is because it shows he was just fine with disappointing you even though he knew the occasion was important to you. THAT is the point.
Yes, he has apologized and tried to atone for his screw up. That much is good. But you need to make it clear that this should not ever happen again. You do not want a lifetime of him putting you on the backburner when he gets a chance to have some fun with his buddies. Nip that in the bud. It's one thing if he didn't have the money and you had to postpone the evening out. But he spent it elsewhere. Big difference.
Nip it. Nip it. Nip it.