Congratulations on your wedding! One thing you will want to keep in mind when planting your flowers is that by September, most flowers will be mature and getting 'leggy.' I would stay with flowers that will hold well through early September. Easy to grow flowers include marigolds, zinnias (my fav), begonias, salvias, sunflowers, and geraniums (gerums) including scented geraniums. Geraniums like cooler weather and will bounce back nicely as September rolls around and be lovely. These may seem like simple, old fashioned flowers, but they are easy to grow, come in a variety of colors, and look good late in the season. Planted in mass, they are quite understated and elegant. There are perennials that bloom in late summer, early fall that are lovely, but if they are small and not established, they may disappoint. Roses come to mind!

Around September, nurseries will start bringing out cool weather annuals like pansies, violas, and flowering kale. You can always supplement what is in the ground with lovely pots of these purchased annuals.

Not matter what you decide, this is your day and your beauty will outshine any flower you have!!!!

I'm in Chicago and had a similar idea. I ended up growing about 7-8 dinnerplate dahlia plants (from tubers). With the assistance of a local gardener who had 1 color that I was missing, I had enough for my bouquet and my 1 bridesmaid. I should have grown some smaller ones (in a sunnier area) in my colors so that they could be interspersed.
For the rest of the flowers (about 11 tables + boutineers & corsages) I purchased roses, etc from Costco. I think it was about $600 + trial bouquets (I just cut them down and put them in loner vases).
It worked, but it is absolutely the 1st thing I would do differently. I pretty much did it all myself, was totally stressed making boutineers the morning of the wedding. Unless you have an experienced group of friends willing to give up their time + a refrigerator or cool space to store it in, I don't really recommend doing this :D.

One advantage to the dinnnerplate dahlia is that you could just carry 1 as a bouquet and put 1 on the table (there is a wedding here that did that (link below). If you want them to be that big, plan for 1 flower per plant that is ready (you'll have to prune others). You may get 2 or 3, but better safe than sorry.
FYI, I had no problems growing the dahlias & it was my 1st time.

I imagine that if you purchased dahlia tubers now and started them indoors, they would be ready by Sept 8, but you could check with your local dahlia club.
If you do, I hope you post pictures though!

Here is a link that might be useful: dahlia wedding on gardenweb

I also advocate for dahlias -- they reach their flowering peak in September and October up until the killing frost. These are tops if you want cut flowers.

If you love purples and blues, go for New England or New Yourk asters.

But, perhaps you're looking for massive vistas in your yard of flowers blooming at the wedding?