Ah Weddings, doncha just luv em!
First thing, very few weddings ever run to time and the people who book you have virtually no experience of putting on a gig. And there are usually kids whose parents like nothing more than dumping them on the dance floor and letting the band entertain them while they, the parents chat to their grown up friends. Very ocasionally things go to plan, no kids, gorgeous women and good tippers. So, assuming the former is the case, just remember that the day belongs to the bride and groom, be very friendly towards the catering and bar people so you'll be fed and watered well, expect the worst but hope for the best, quote a good fee, smile and put on a good show - you might get return (hopefully non-wedding) bookings.