MUST HAVES:
A suitable bag: I carry everything I need in a shoulder bag or a bag that's about the size of an A4 sheet of paper. You'll want a bag that's big enough to carry everything you’ll need, that’s small enough to fit the festival or venue restrictions.
Phone: This is the most vital thing that you’ll need to pack for any festival or concert, so you can keep in contact with friends and family (as well as people you're with - in case someone gets lost). You’ll also be able to take photos and videos, which is a must for me.
Battery pack: To go hand-in-hand with your phone, I would recommend bringing a power bank (battery pack) just in case your phone dies. Phones can run out of battery quickly, especially at a concert or festival. When you’re on it basically all day, it’s bound to run out of battery.