Rehearsal Dinner - What to wear?

[Cardinal026]

I know, I know, another wedding question, I'm sorry! But the advice I've gotten from friends and family is so varied, I thought I'd turn to GC...

I'm going to a wedding in Pensacola next month for a cousin of my boyfriends. Even though we've been dating 6 years, I've never met anyone on this side of the family. Since we're not from that area, we've been invited to the Rehearsal dinner, and I have NO idea what to wear, and theres no one to ask, since I don't know them either!

The wedding is a 5pm on a Friday wedding, and judging by the invitations (very elegant) I have a just-below-the-knee silk cocktail dress...but would a Thursday evening rehearsal dinner require a dress just as fancy? Or as casual as a suit that I'd wear to the office?

Thanks for your help