Harry wasn't going to go to that party... But then the screw up and he lost the Sword in a silly attempt to kill Lea with it. So he and Michael set off to try to recover it at the party..
Which was way complicated by his discovery that Susan had crashed the party, which was an added distraction. It almost seems like another hand was at work here... Think about it, Harry, knowing it bode no good for him to go, had no intention of going to that party. If he hadn't lost the Sword, he never would have gone to the Party with Michael, which set off a whole list of things.. Interesting don't you think that after when all the dust has settled, I think it was Justine, turns up at his place with the returned Sword, totally unharmed. 1] Harry had no intention of going to that party. Would he have figured out that Susan and stole and forged a copy in time to go and save her? 2] Even if he did find out in time and went, without the Sword being there, there would have been no attempted sacrifice with it, which is what really set things off...
Susan is a distraction in all of this, the shiny object that we all follow.. But in the order of events at that party, she is of no consequence. Yeah, I know she gets half turned, her and Harry have sex, little Maggie is the result, which leads to the downfall of the Red Court.. Important, but....
What is important? Harry panics and makes that silly attempt to kill Lea with Michael's Sword.. It doesn't allow Itself to be misused, but instead of shattering,[as the Sword of Faith did when Murphy misused it,] if merely falls out of his hands and Lea grabs it... Convenient, don't you think? Yes, Murphy was an ex-Holy Knight, Harry, just a punk wizard, might have made the difference between shattering and not, but still.. Suddenly Lea has something to present or trade at the party.. Harry and Michael know that has to be prevented so they go to the party.. Events unfold, Lea is presented with the infected Knife, an attempted sacrifice of an innocent with the Sword, which really triggered the chaos. Important thing to remember without the Sword falling into Lea's hands and going to the party, Harry wouldn't have gone to that party, and would have remained ignorant of a lot of things that become vital that he know for the rest of the series.. What is even more interesting, for all the events that should have shattered or altered it, in the end the Sword is returned to Harry's place and thus to Michael, totally unharmed... It is almost like the Sword had a mind of it's own, and it does, if one thinks about an angel possessing it.. Another Hand needed for Harry to be at the party, to get vital information for the events to come, and manipulated him..