Jack has of course dressed up wholeheartedly for the occasion and can be found almost anywhere in the dome at some point in the day, alternating between commenting on lack of historical accuracy ("Hey, I've been to the Renaissance!") and plunging into all the most fun parts of it, including eating, drinking, fencing, and of course spending quite a lot of time both watching and indulging in umpteen battles of rövkrok. (What? You didn't see that coming? How could Jack Harkness resist a sport that translates as 'ass-hook'?!?)
And of course once the evening comes, he can be found at the dance, working his way across the dance floor as well as spending some quality romantic time with Ianto.