This is the Coke Christmas tree commercial from 1977 which I remember so very fondly from my childhood. The song is "I'd Love To Teach The World To Sing," and it reflected in a beautiful way the spirit of the season.
Heres a 1978 version of the commercial. Same song, a little different.
Here is the original classic commercial which used the "I'd Love To Teach The World To Sing" song. Entitled "Hilltop," this original version was not strictly Christmas related, but rather was first released on July 1971. Its message was one of hope, and featured young people from all around the world assembled on a hilltop in Italy, each holding a bottle of Coke from their respective countries. The song "I'd Love To Teach The World To Sing," with the reference to Coke removed, later became a big hit for The New Seekers and a studio group called the Hillside Singers.