Answer: It's very difficult to predict exactly when certain plants will be in bloom. I suggest you consider planting a variety of early-summer flowering bulbs. You can plant a mixture of different types of lilies, for example, this fall. Try to choose a mix of early, mid-season, and late varieties, for the best chance that some will be in perfect bloom at the right time. Also, consider planting some irises (bearded and Dutch).
Next spring, I would purchase transplants of a mix of the following: petunias, geraniums, impatiens (partial shade), marigolds, snapdragons, zinnia. You can start some from seed, but I think you'll be more assured of having flowers for the wedding if you use transplants, unless you can get the seed started pretty early.
You might contact the Pennsylvania Horticultural Society (http://www.libertynet.org/phs/) and see if they have any info on when various flowers tend to bloom in your region.
Good luck, and congratulations!
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