take 2 changes of clothing. wear casual dress before the ceremony that is more breathable and let your client know that you plan to change before the ceremony starts. That way you can wear cooler clothing, still be professional, change halfway through the day, and look presentable. Most clients dont care if you are more causal for the getting ready portion.
If youre afraid its going to be blazing hot, take a lunch cooler, fill it with ice and some washrags. As needed, wring out the wash rags and put them on your neck to cool
Also take bottled water and plenty of deodorant.