Timing on these things is always an issue - too early and many annuals will have bloomed out and gotten lanky by August. Having done a number of wedding containers projects (including a few fast jobs only a week before the big day!), I'd recommend you wait a bit before planting, like around Mother's Day. This will give you time to start from seed if you wish or wait and get starts at the nursery.

As to plants to include, you want to consider how they will fill in and and how long they will bloom. And you want a range of heights and textures, just as you would in any seasonal container planting. Million Bells (Calibrachoa) are very long bloomers, are self cleaning(don't need to be deadheaded) and come in a range of colors, including various shades of pink - they'd make a great filler plant. You might want to include cosmos, another very reliable annual, but stick to the compact forms - many will get too tall for your purposes. Geraniums are always good and will bloom until frost. And consider other colors besides pink - white is a great accent or pale blue or even a more intense magenta pink. Lobelia, salvia and petunias will all be in this color palatte and also offer a long period of bloom. And don't forget foliage effects - maybe coleus or creeping charlie or variegated ivy to trail over and soften the edge. If you get them now and pot them up in gallon containers, Stargazer or Casa Blanca lilies will typically bloom in August and will add fragrance and dramatic impact to your containers - just slip the lily, pot and all, into the center of your container and plant the fluffy stuff around them. And I recommend overplanting a bit to make sure your containers are full and lush - nothing looks worse than a stingy presentation.

Use a quality, well draining potting soil, add some slow release fertilizer (osmocote) when you are potting and use a dilute liquid fertilizer frequently during the season. And keep up with the watering and grooming so they look their best for the big event. I've found that keeping these containers in only part sun will allow them to develop well and fill in fully without blooming too early. Then several weeks before the wedding, move them to where they receive full sun to prompt a full flush of bloom.

Thank you so much for your advice. I had completely forgotten about geraniums...and the greenery! My brain is so focused on other things right now. Making my list for spring flower shopping.

Pesky1 - sorry but whenever I read about people wanting to grow flowers for weddings or other special events my first thought is why add stress to the big day? There are too many variables that could go wrong and cause disappointment then you'll be scrambling at the end to find replacements. If you are really set on doing plants, one thing I could suggest to you is make up silk flower arrangements ahead of time. They have really 'life like' flowers now that resemble geraniums, summer flowers, etc. That way you can make ahead and have for the big day. Check out Michael's Craft Store, AC Moore, or JoAnn Fabrics.