NTA. You asked your boyfriend to take the date off your five year anniversary off. He knew that this mattered to you. Then a week before the actual event he offers you the day before, but on the actual anniversary wants to go to a party you can't attend and get hammered with his buddies. So NTA. It would be a different verdict if you could attend. But he's not only breaking your plans for your five year anniversary, but doing so in favor of a party you can't even attend. If I were in your shoes I'd dump his ass. He sounds like a loser. I'm an attorney. There are plenty of high stakes social events I get invited to. But I would never dream of leaving my partner home, alone, on our five year anniversary while I get sloshed with the fellas. You deserve so much better. I truly feel sorry for you. NTA, NTA, NTA.