You don't say what the exposure is so it's a bit hard to recommend. If it's a sunny area Roses would be blooming at that time of year. If you have some shade Hydrangeas are a beautiful summer bloomer.

I am having a wedding here on Saturday! (my son). Right now I have blooming lots of black-eyed susans, coneflowers, hydrangeas, cannas, lantana, hollyhocks, yarrow, daisies, astilbe, gallardia, monarda. my roses are taking a break, they were great in May here. I live near Tupelo, so that gives you an idea.

Ilikemud- my yard is ALL sun. No trees at all and we own 20 acres, so plants that take up a lot of space are no problem.
Thank you Louisianagal. That was all really helpful. I have some rose bushes and two hydrangeas and most of that other stuff I think I can start from seed. Thanks! Oh... is it too late to plant seeds?

It can be very tricky to plant for a specific date since the weather can create so many variables. Your best bet is to plant a wide variety of perennials, with a fairly good number of annuals for insurance. I am linking below to a list I gave in the perennial forum of low growing plants that have bloomed in my garden since the first of the year. I live about halfway down the state of MS on the far eastern border. The idea of roses and/or hydrangeas is excellent. You need some large plants scattered throughout your bed for mass.

Here is a link that might be useful: My garden, Feb to June 1, 2010

If you double click on the link below, you will see some pictures of my garden. You'll get full size images if you click on Slide Show at the top.

Here is a link that might be useful: Donna's Garden, 2010

My first school was grades 1-9 and then we went to high school. Every year we had 9th grade graduation. I'll always remember the graduation flower arrangement. The florist always donated the arrangements since she was the only florist in the surrounding area it was good PR. She used two large clay urns and cut Queen Anne's Lace to put in them; no greenery. Since out school colors were red and white, she used a spray to color some blooms red. You could use as many colors as your want. Queen Anne's Lace makes a very lacy, ethereal arrangement, well suited for a wedding. Best of all the grow wild and are free. How about tall, pink glads surrounded with Queen Anne's Lace?

Not sure what zone you are , would hydrangea be bloomimg during that time by you ????They make beautiful bouquets.I was going to do that for my my son's wedding and my own , did not happen and do not count on this !!! Plan an alternative if you do insist on trying ! I used silks for the tables, expensive but they have coupons for 50% off and when you open a new account online ...plus you do have 2 years so start buying now and get creative .I bought fillers very cheap at the dollar tree! They turned out great.Also piece I put above the arch costs me $150.00 at least in materials . but oh was it gorgeous ! My mom has it above her fireplace ! It would have cost well over $1000.00 for fresh flowers /labor at some florist !!!! I did by other flowers (fresh) for corsages etc.. at Flowerama , and the cost was alot less then some of the more popular florists in town , but they let me look in the cooler ,were very helpful ,and in the end I got some beautiful stuff and you sure could not tell it did not come from a more exp. florist ! Also my wedding was in the Fall, silks have a great fall collection ! I had also a chocolate 3 tiered wedding cake with chocolate cigarellos(chocolate tubes , shaped like long cigarettes?) going all the way around each layer and fondant around it acting as if it were holding them on the cake , anyway the cake had real flowers all over each layer and surrounding the bottom and on the top was a beautiful assortment...I just went to all these places till I found the colors I was wanting,and they let me buy just a few pices at the florista I went to,mostly went to like Publix etc.....plenty extras but better to be safe ...OH And each layer of my cake was a different filling ! Chocolate cream, cappuccino, rasberry ! SO YUMMY!

Congrats! You're smart to plan ahead.
Are you wanting flowers to cut for bouquets or are you wanting flowers in the ground for an outdoor wedding? Or both?
Since you don't know what colors, I'd plant mostly white flowers, roses, daisies, etc.
You might want to look through the forum for Cutting Gardens.

Here is a link that might be useful: Cutting Gardens