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In what part of the country is the wedding? I think the NYC area would be dressier than most other parts.

Because I wouldn't want to go out and buy something new, I'd wear the dressiest dress I have, which would either be a cocktail length black halter dress (slightly shimmery fabric, with a brooch kind of detail at the neck) or a knee length satin-y strapless blue dress. I'd dress either up with my nicest shoes and jewelry.

I don't think you need to go nuts getting something perfect. I'd wear the fanciest dress you have and go from there. Men should wear dark suits and ties, or if they have a tux and want to wear it, they could do that but it isn't necessary.

Formal just means that tuxes aren't required and that cocktail-length is still appropriate. If it were me, I would opt for a long gown, and despite the fact that the ceremony is a late afternoon ceremony, I would gear my dress for the evening reception.

ETA: For the last formal wedding I attended, I wore a one-shoulder floor length satin gun metal grey gown (ETA: with a high-ish slit up the front so that you could see mah shoes!).

@Mrs. Lemon-Lime: I would wear your nicest cocktail dress (black is always more formal to me) or a floor length gown if ou have one as it sounds like that is what she's into. I would but DH in a black suit with a white shirt and tie.

I'd try to do a floor length gown if possible but not a heavy fabric since it is Miami. Rent The Runway would be a good option for something like this if you don't want to buy a one time dress that you might not rewear. I'd wear something like this: http://www.renttherunway.com/shop/designers/halstonheritage_dresses/lightsioanninamaxi
or this:
http://www.renttherunway.com/shop/designers/nicolemiller_dresses/yuyuangardenmaxi

I would ask the bride what she means/wants by "formal." Formal as in black-tie or just more than cocktail?

I've done knee/just below knee length dresses and DH in a suit to black tie.