Even if you are planning a summer wedding always have a contingency for wet weather. You may have the idea of drinks and photographs outside but do think about where they would be if you had to inside. Maybe have a basket of umbrellas on hand at the church or venue and willing ushers to escort guests in case of a downpour. Manage your expectations well before the big day, embrace it and remember photographers actually prefer an overcast day as it can eliminate shadows, and can result in some incredibly moody and romantic images.