Firstly, make sure that your parents are in a good mood. This way you know that they have a clear mind and are willing to think about things positively.
Prove to them it will be safe with brochures, a website, etc.
Tell them, "who, what, when, where, why, and how"; that is: who you are going with and who's performing, what type of festival it is, when is it, whyyou want to go and such, where it is, how will you get there safely and get back home safely.
Explain all of this to them and make sure they have a good understanding of it all.
Ask a few days in advance to give them time to think about it.
You could also try to make a deal with them, such as, you won't go to any other concerts until next year, you'll pay for your own ticket, you'll text them or call them frequently so they know you are safe, etc.
Be mature about it, and understand what they are saying to you. If they don't allow it this time around, don't fret too much, and show that you can handle it, so next time around, they'll trust you as being mature enough to go.
