I notice some people are saying things along the lines of "fuck it, you're an adult. You don't need permission." But really things are not that simple. Especially if there's a cultural reason behind it. I have been in the same situation as you so I wanted to offer some advice I have used on the past.

Tell them the truth but the bare minimum. I would tell them that you are going to a concert for a few days. Do not show them videos online but show them the pictures inside of the booklet that came with the tickets so they get a feel of what an amazing experience this will be without showing them images that might be too scandalous.

Tell them that you are going with a group of MULTIPLE friends, not your boyfriend. And that there will be a room for girls and a room for the boys in the group.

I'm not sure how you plan on getting there but my parents felt a lot better when I let them know that I was not driving my own car out there. I also planned outfits that were more modest to take pictures in (shorts and a cute top) so you can show the pics to your parents and then outfits that show more skin to wear once I'm inside.

If you are doing something along the lines of a greyhound or bolt bus, they might feel better that you have a safe method of getting there and back without leaving you with the responsibility of driving. I know my parents felt better about that. I also made sure to call them once I arrived in Vegas and once a day everyday to let them know I'm safe.

Again, I'm not sure exactly what your situation is but this is what has helped me in the past :)