TIP I: HYDRATION IS KEY
First things first, be sure to set up Fancy Water or Hydration Station(s) for guests to self-serve (keep it eco by skipping plastic bottles or cups) For my own social parties I always have sparkly waters available to grab near where the booze is being served.
For really hot days I suggest having a Misting Line setup somewhere too so guests can give themselves gentle bird baths in the mist & cool down.
TIP II: USE YOUR DECOR & ENTERTAINERS TO ENHANCE GUESTS COMFORT IN THE HEAT
Starting with the obvious here is plenty of shaded areas. Umbrellas, tents, & shade coverings.
I have seen thoughtful planners give out cooling Gifts like sunglasses, fans & parasols. We always bring spritzers & fans to any hot Summer bookings & give them to our performers to offer a quick refresh moment to guests. This can go a long way to making guests feel considered & cared for on a hot day.