This is the packing list I use for camping festivals while trying to pack light for carpooling or flying. It’s somewhat minimalist and leaves out extras, while still including all of the necessities, and some conveniences and comforts. It assumes you are at a regular camping festival with typical amenities such as water, food for purchase, and bathrooms.
I’ve never felt like I was missing anything I needed with this list, and I try to refine it after each festival to remove any unnecessary excess. You may need different supplies for certain specialty festivals, such as those with extreme weather or limited access to standard festival amenities. (For example. Burning Man has an entirely separate list). Read the festival website carefully to see if there are any special concerns or recommendations for that festival.