One of the big festivals to happen each year in the U.S is The Bonnaroo Festival which happens each June in Manchester Tennessee.  I have never had the chance to go there but from what I've heard it's one of the best music festival experiences there is.  Elton John, Jack White, Kanye West among the headliners.  Check out Ultimate Classic Rock for more on the line-up and how you can get tickets to be there.