Maybe look at plant selection guide near front of Sunset Western Garden Book. You can also find summer bloom times and suitability for container culture, cutting for arrangements mentioned in other publications (and web sites).

There is a whole set of garden plants that bloom mostly before July, and another set that bloom July to October. If the wedding will be late enough in the month this second, long-blooming group may be the one to draw from. The spring bloomers (July is the start of summer here) often do not bloom for a long time, the summer through fall group will be more likely to correspond with the wedding date as many of these are "ever-blooming" - once they start.

How about hydrengas? You can even start dahlias in pots in a protected place to "help them" bloom earlier? I have done this before.

I highly recommend annuals (and a few perennials that bloom all summer like pelargoniums/geraniums). You can buy them at a reasonable price early in the summer, about a month or so before the wedding, and pot them then with plenty of time for them to grow and settle in to blooming beautifully. Why add more stress to an already stressful time?