AITA for being angry my boyfriend ditched my best friend’s engagement party for a spontaneous concert?

I (26F) have been with my boyfriend, Mark (27M), for three years. Last night was a huge deal for me: my best friend since kindergarten, Sarah, was having her engagement party. Mark and I had RSVP’d weeks ago, and I was so excited to celebrate with her and introduce Mark to some old friends.

Mark is introverted, and he usually prefers quiet nights in, but he agreed to go. I bought a new dress, did my makeup, and was really looking forward to it.

About an hour before we were supposed to leave, Mark got a call. It was a group of his old college friends. They were all in town spontaneously because their favorite band from university, which rarely tours, was playing a surprise pop-up show. Mark was incredibly excited; he rarely sees these friends, and this band meant a lot to him. He asked me if I would mind if he skipped the engagement party to go to the concert with them.

I was instantly hurt. I told him this was important, that we had committed weeks ago, and that it would look terrible if he didn’t show up. He argued that this concert was a “once-in-a-lifetime chance” with friends he misses, whereas Sarah is my friend and I would see her all the time. He promised to take us out for a nice dinner to celebrate later.

We got into a massive fight. I accused him of prioritizing a random concert over a significant event in my life and over a commitment to me. He said I was being rigid and unsupportive of his rare opportunity.

In the end, he left for the concert. I went to the engagement party alone, feeling embarrassed and having to make up excuses for why my boyfriend couldn’t make it. The party was lovely, but I spent half the night trying not to cry. When Mark got back, he was buzzing from the show but immediately apologized when he saw how upset I still was. He maintains he didn’t see it as a huge betrayal, just a tough choice.

My best friend Sarah said she understands why I’m mad, but another friend mentioned that since we are long-term, maybe I should have just let him go since he was so happy.

AITA for being furious that he broke our promise to go to a concert with his friends?