First off, I hope you have a good pair of boots to go with your sundress, because there is some yucky rain and mud going on!
But secondly, maybe you should consider staying cool with some Mint Julep Ice Cream from Simply Recipes while you’re there. Wikipedia says, “Traditionally, students and other attendees dress in a southern, aristocratic style often seen at other steeplechase and horse racing events. Seersucker, bowties, and pastel colors predominate the atmosphere, particularly at the spring race.”
This would definitely complete the look. 😛