Do you have all the ingredients? You can scroll down through the list of unavailable ales, then find the one that is marked Feast. I think it needs honey, along with a couple other items. So go to the Village Shoppe and pick up the ingredients you need, then go back and hope that the bride and groom are still around! I thought for sure I was going to fail that quest because as soon as my couple married, they went home (and I couldn't cancel it). But then they went back and I was able to get them their ale. I think I ended up having to brew it twice, so you might want to get extra ingredients.