For black tie and black tie optional events, Emily Post deems a formal floor-length evening gown or a short, dressy cocktail dress appropriate. Formal weddings have a similar dress code: The Knot says a long dress or a cocktail-length dress is appropriate. For these, get a cocktail dress.
The difference is in the material -- think chiffon, silk, satin or taffeta.
On the high end, BCBG has a flattering two-tone black-and-white cocktail dress with a sweetheart neckline. It costs about $318. For a trendier look, try the one-shoulder draped "Goddess" Rachel Roy dress. It costs $109 at Macy's. The only rule in color with cocktail dresses for weddings is to avoid white or off-white.
For beach formal or semiformal or casual weddings, get a formal sundress. A Donna Morgan sleeveless cotton sateen sundress in a light yellow-and-black floral print ($99, Dillard's) is a good buy. It has all-figure flattering details: a sweetheart neckline, belt at the waist and full skirt.
Another cute dress that would work is a purple Max and Cleo wrap dress ($118, Dillard's).
Both are bright enough for daytime weddings and would translate well to a night wedding.
Looking for something cheaper for formal weddings? Don't try to save by choosing a maxi dress or jersey-material dress for a formal wedding -- you'll stick out in the wrong kind of way. Try secondhand or vintage stores. You may find a one-of-a-kind outfit.
For a cheaper option for a dressy sundress, go the secondhand route or update something you have with new accessories. Try a nautical-style rope belt, beaded bracelets, embellished sandals, or for a daytime weddings, a straw fedora.