What kind of music are you into? And do you have a plan of where you want to go, or will you follow the music?
In July, there's a big music festival in one of the three Baltic Sea states - Lithuania/Latvia/Estonia - I think it's in Estonia this year, though I may have done my math wrong. It's traditional regional folk music, and there are usually umpteen choirs and about 10,000 singers/musicians/dancers that participate. Jazz is also huge in the Baltics, and Vilnius has a great summer concert series - not just jazz, also classical in the big venues, but lots of rock and some indie in the smaller pubs and clubs. That one's called the St. Christopher Festival - but it's basically just the summer concert series, as it goes on for months and there's something going on anytime. Also, there's the annual sea festival in Palanga in August.
In June there'll be midsummer festivals, the ones in the Baltics have very strong pagan overtones, since they were the last Europeans to convert, and the "old ways" are still very much alive.
This year, Austria has turned into a year-long Mozart festival - anywhere you go, at any time, there will be something Mozart-y going on.
World Cup in Germany will be turning the country on its ear for most of June, so there'll be plenty of young people, drink and partying throughout the country during that time. I imagine there'll be a concert or two, taking advantage of the imported audience...