A wedding guest's only job is to show up at the ceremony and reception on time, smile and wish the happy couple well. You don't have to do anything else unless you've been specifically asked to participate in another way. However, there are a number of things you should never do at a wedding:
1. Don't change into casual clothing halfway through the reception.
This goes for children too. Yes, sometimes it's hot and sticky at an outside venue, but at the end of the day, you and your family will show up in the background of all the photos looking dressed for a picnic instead of a wedding reception. It's unfair to the bride and groom who've paid for expensive professional photography and gorgeous décor.