Most of your guests will feel at ease seeing "semiformal" on your invite. This indicates the kind of attire they're most familiar with for evening weddings: cocktail or festive, dinner party-appropriate attire they can find in the eveningwear section of a department store. We're talking knee-length, "big-night-out" dresses of the sort they might wear to a New Year's Eve party or on a special date. "Semiformal" is the reason why the little black dress exists, although at a summer wedding, color feels more seasonally-appropriate (from pastels to jewel tones). Men in the business world also have an easy time with "semiformal" as they can wear a suit-and-tie combo appropriate for the office. To put a festive spin on the standard, men might also consider pairing shirts (how about some stripes?) with vibrant and/or patterned summer ties.