I've taken my child to about 30 outdoor festivals and we've never, ever had a problem. Obviously, it depends on your child but DC sleeps in the sling and I've always been able to carry her for long hours. I took DC to Jazzfest in New Orleans (intensive festival experience) at around 9 months and it was awesome! I had like 5 people taking photographs of us. We also went to a overnight festival when she was about 12 months (Reggae on the River). Also, we've been to lots of evening shows and festivals with no problems. DC falls asleep *easier* to loud bustling vibes and is totally safe in the sling ~ I can even dance with her sleeping.
People at festivals love babies and the pace of the day seems to work well for most children. Can you tell that I think you should go? LOL! Now, be aware that the experience is totally different with children. You simply don't cram into the third row, you don't 'party' like you might have and you don't get to see and do everything. But, you always have someone to dance with! Festivals are my absolute most favorite thing to do with my child. Day, evening, family orientated & and not so much, music, cultural, food…anything - I'm there. Go, you child will be fine…and if not, you leave. What festival is it? Maybe I'll come, LOL!
If you decide to go, bring some sun protection, a cloth to get wet, water, a blanket and a sling/carrier. (oh, and make sure babies are welcome and don't need a ticket!)