For Men: The most relaxed suit you own. We're talking a lighter-hued (i.e. khaki, beige, buff, fallow, or whatever your favorite name for tan is) cotton or linen suit paired with a lightweight dress shirt, also in linen or cotton. Leave the tie, but not the shoes—formal equals footwear, always.
For Women: A formal summer sundress at tea or knee length with flat sandals or wedges. Makeup and hair can be natural.