Clematis won't grow up your posts without something to cling to, so I'd plant them just to the right and left of the posts so they can grow on the chainlink. Get the biggest clematis you can, most probably from a local nursery. They can probably advise you what will be most likely to be blooming in late May, though they can't guarantee what will be blooming as mother nature isn't consistent. Ask in another post in this forum about fertilization, pruning and watering so that you can take the best possible care of your clematis.

Have a backup plan: Silk garlands with leaves and flowers or wiring vases filled with flowers onto your posts or whatever.

Look into planting sweet peas on the fence between the clematis. It's an annual, early blooming nonedible pea vine with flowers that come in purples through reds, pinks and whites. You might be able to find a local greenhouse that would be able to start the seeds early for you so that they will have more size by the wedding. Plant a variety of plants to increase the likelihood that something might be blooming during this time.

I think you can def. accomplish it if you get some in the ground now. My second year clematis from silver star vinery have been pretty impressive.

Guaranteeing that Clematis would be that tall and blooming in one year is not something I would do. I would strongly suggest considering annual morning glories or sweet peas as a fill in if the Clematis do not perform as you want. Clematis are not instant gratification plants. Most do not do their best until they are in their third year...

What color are you looking for? Some varieties are more rapid growers than others....