A free concert by the Chicago Symphony Orchestra is a rare occurrence, and always a popular one. You can catch the CSO this Sunday (Sept. 11) at 4 p.m. in a 9/11 tribute concert at Pritzker Pavilion in our still-new Millennium Park. The program of favorites will offer music by Americans Charles Ives and gay Aaron Copland and by Russkie Rimsky-Korsakov. The conductor is William Eddins. If you haven’t been to Millennium Park yet, the Pritzker Pavilion is the thing that looks like the spaceship from Alien. There’s plenty of room for a picnic on the Great Lawn.