As a fellow C's fan who only gets the luxury of seeing the Celts in person once a year (Portland), I can offer my perspective if you're still not 100% committed to ditching the game for the wife's party:
It's just a game. I missed the Celts visit to Portland last year and it didn't change my life - no big deal. Don't forget, there's always next year's game. If you save the money from the tickets to this year's game, you could have double the money for next year's game - maybe you could get some real primo (courtside?) seats. I did that a few years ago - splurged and got courtside seats when Boston came to town - I'd rather do that once and watch the rest of the games on TV than get decent seats every year.
As a guy with a spouse that isn't very much into sports, you don't want to make sports a wedge between you and her. Your situation is the perfect example - if you go to the game, it will become a wedge - and wedges are only good for splitting things (think chopping wood).
Marriage is a two-way street. She can't always have it her way. But it sure sounds like this is a case where you should give in. She'll appreciate you for it.