I don't know if many people realize how important Joni Mitchell (63 today) has been in the development of popular music. For example, I know that I don't. But I'm learning; apparently, she's an uncompromising, constantly evolving artist, probably the most relevant female musician of the last half of the 20th century. I might quibble with that -- Madonna and Aretha Franklin would have to be bigger -- but I'm willing to hear a case. What was a late-'60s concert festival that turned ugly, prompting Joni Mitchel to plead with the crowd to stop booing?

York

Isle of Wight

Jamestown

Columbus

Altoona