The invitation says garden party. The venue is a chapel at the top of two hundred stone steps, the reception is on a beach and the forecast says wind.
Start with the shoes
Flat, or a block heel at most. Stone steps and sand will defeat anything thinner, and you will be dancing on both.
Then the colour
White belongs to someone else that day. Island light flatters strong colours: green, coral, deep blue. Pick one and keep the rest simple.
Bring a layer
Sea breezes turn cold after sunset. A light jacket or a large scarf will be the most borrowed item of the evening.