This song is always trending on Twitter every September, every year. So basically, it's a Green Day song. I still love that song even though it's quite old already, it really never gets old.
This song is about the band Green Day's lead vocalist Bille Joe Armstrong's father who died of esophageal cancer when he was just 10 years old. It happened on 11th of September. They were very close and his dad bought his first acoustic guitar and would travel around the hospitals singing to other patients. The day of his funeral, Billie ran away and trashed his room. His mother saw him, he cried and yelled, "Wake me up when September ends!"
The song lyrics is really accurate. Others say the song is about the war but Billie have said in the interviews that it was really about his dad.