You might want to consider annuals in pots for a garden in August. Phlox and brown-eye susans are some of the perennials that should be blooming in August. Good luck!

In my garden still blooming even yet is
Dahlia (they get real bushy and big with gorgeous flowers)
There are 1000s of varities.
Portulaca
Petunia
Geranium
Russian Sage
You can buy some already flowering mini roses at the super market and give them out as favors, put them on your guests tables, they will look nice.
You can also buy some cut flower bouques at a place like Produce Junction and put them in baskets where you are to be married.
Or for a tropical feel, buy some big hibiscus already in pots and put them in the yard for pics.
Good Luck!
Just be sure to have plenty of shade available for your guests alsong for yourself, August is very hot and humid.

Black eyed susans are very easy to grow and, if planted as seed next spring, should be in fine form next August. Zinnias, Salvia coccinea, and Cosmos are additional choices which can be seeded in the spring for a strong August bloom. All of these can be grown without too much care, but you'll get a bit of an informal look, since all get fairly tall. I might mix the four in large beds for a nice english garden sort of look.

Many tamer (smaller) annuals could also be grown in beds in the ground or in pots. Marigolds are easy, garden sage usually peaks in August. Pansies and perhaps petunias should be avoided, because they may tend to poop out by August.