I'm going to the Rock Steady Musical Festival in England, next July. I'm taking suntan lotion, a sleeping bag, backpack (rucksack), T-shirt, shorts, and some bottles of water. There will be different stages playing many types of rock, dance, pop, and other music styles.The ticket price is 49 dollars (or $49) which allows you to see all the bands.
It's very unlikely that they will let you take in bottles of water due to the franchising contracts.
Don't forget to take your sunglasses and a raincoat.