Daisies, daylilies, blackeyed susans, blanketflowers, roses will all still be blooming in my garden this week. You could plant this year and hope for some good flowers next year. But perennial plants are expensive, so only do it if you want them beyond the wedding, otherwise it won't be cost effective. I don't think any annual flowers from seed would bloom in time, and perrenial seed usually takes more than one year to bloom. I am growing Zinnias for a friend's wedding this fall, and am hoping that works!

Roses and more roses.....and still more roses...plant them as soon as you can this year and hope they will produce something by next year.
Then contact your local garden center and tell them you will be wanting lots and lots of started annuals to plant in your garden as soon as all danger of frost is past...after may 15 in your area.
Then buy things like rocket snaps, bachelor buttons, petunias in the colors you like, Zinnia are iffy because they get a powdery mildew that often makes them ugly.
I have a friend who bought all the white geraneums in the spring for her daughter's late June wedding. She had pots all around and on tables and gave them as favors...got lots of use out of the plants.
Linda C