Go for a look that is upscale and tasteful but still suitable as daytime apparel, such as a stylish day dress or a suit. Luncheons are typically daytime formal events.
When choosing an outfit, consider the reason for the event. At a business or networking luncheon, formal business attire such as a skirt suit or a sheath dress are appropriate. For a social event or a charity event, flowery dresses, flouncy skirts and pretty blouses are a suitable option.
The keys are not to show too much skin and not to over-accessorize. Keep the look fresh and simple, with either neutral beiges and whites or brighter colors.