"It's a Beautiful Day" is a popular song by Canadian singer Michael Bublé. It was featured in the Season Seventeen episode, "World War Zimmerman".
Background
Eric Cartman has a nightmare that he is Brad Pitt in the movie, World War Z. He re-enacts the breakfast scene and the traffic jam scene. When the family asks for the radio to be turned on, they all sing along to "It's a Beautiful Day".
The song is interrupted by the news announcement of the acquittal in the George Zimmerman trial. An angry mob of African-American citizens begin to riot, mimicking the zombies from World War Z, in retribution of the verdict.