If it's in August, you might get away with not wearing a strictly black suit/nice tie, but nice slacks and a blazer, with a tie.
If the wedding is huge and formal, you better dress up. they spent a lot of money on the event, you should at least dress accordingly.
If the wedding is less formal, please be dressed up for the ceremony at least. It is respectful. I know someone who's ceremony was formal, but held her reception outside, and asked that everyone please change into more comfortable clothes. the reception was all mexican food.