I'm learning (very slowly -- it may take a few years . . . but I'm actually making pretty good progress) how to play this song on the piano. The chords in the intro are so much fun, and you can see in the video that the band's audience thinks they are, too.
This song was #3 on Billboard Hot 100 in 1972. It was written in 1971, after Robert Lamm went for a walk in Central Park in New York City on a summer day. There were musicians and other entertainers there, and after absorbing all the hubbub, he wrote a song about it.
The band playing the song is Chicago, a rock and jazz fusion group. Besides guitars, drum, and keyboard, there is also a trumpet, trombone, and saxophone. The sound is exciting; it is full of energy and life, but the music also carries the listener right along with it-- it is pleasant and not overpowering.
The song "Saturday in the Park" was on Chicago's 1972 album, Chicago V.
Trivia about "Saturday in the Park" -- did you know it is played for Saturday afternoon baseball games at three different large city stadiums? (Chicago, New York, Denver)