You will get a fair amount of snarking at this site. My recommendation is that you look at as many wedding photographs as possible before the weekend and be sure you know how to set your camera controls to work in different light, especially how to handle low light. If using a flash, bounce it. Ask what they want to be sure that you capture and focus on that. Good wedding photography sites will help you know key moments that have to be captures. Make a list and check it off. Don't forget formals, boring but necessary and rarely posted. You might even buy a book that lists the basics needed. Be sure you have extra batteries and more cards than you think you would ever use-at least 8 gig. Backup stuff would be great if you can pull it off.