Have an early dinner, like 4ish, and take a nap before the show. Shower and doll up like it's a new day, have a light meal with lots of fruits and veggies (carb up after, not before. Idk when metal shows get out, but the 6-7 AM post rave diner trip is a treasure) and drink some Gatorade (especially if you're gonna be drunk or otherwise inebriated, stay hydrated, bestie).
To add, if you’re not drinking, take a pain med (Tylenol, neurofen, etc) before as well, to help your old ass bones and joints to not be sore. I usually start hydrating heavy a day or two before. I usually graban Organika or other electrolyte powder instead of Gatorade, and grab a b12 and Coq12. Oh and comfy shoes! Nothing worse than being at like a second or third day of a festival, unable to stand because of sore feets!
Electrolytes are what's important, yeah. I personally take Brode electrolyte multivitamins whenever I'm "going hard", but it's easier to recommend people to grab a Gatorade.
I just saw yesterday that one of my favorite bands is playing nearby this summer with a 3PM start time!
Then I saw there are 6 bands in the lineup and they don't even tell me what order they are playing in. I don't want to watch 6 bands in a row, 2 or 3 is plenty, then let me go to bed at a reasonable hour.