This was the second single to be released by Florence + the Machine, the recording name of London-based alternative pop artist Florence Welch and her back up band.
This paean to the inevitability of happiness was inspired by a giant text installation (of the same name) by the artist Ugo Rondinone, which Welch used to see every day riding her bike over Waterloo Bridge. Florence told Mojo magazine April 2012 about the rainbow-hued text: "It was plastered over the south Bank in London for six months and I rode past it on my bike every day. It's a reference to the dog star, Sirius: when it was closest to the Earth, all the animals would get languid and sleepy; when it moved away, they'd wake up. I've tried to get in touch with him to say thanks."
Welch told CultureDeluxe that most of the sounds on this song, "came from a tiny Yamaha keyboard." Also the percussion sound was "hands on the wall while hitting a drum underneath it at the same time and the beat was really accidental." She added to BBC Newsbeat: "It was just a song that I did at my friend's studio in Crystal Palace [south London]. We didn't have any instruments and we were in a studio the size of a loo."
The song was used in an episode of the Channel 4 teen drama Skins.
The song failed to reach the UK Top 75 after its initial release in December 2008. However, following a performance on the New Years Eve edition of the Jools Holland show and its use in an advertisement for the TV debut of Slumdog Millionaire, this charted for the first time in January 2010.
Welch told the NME January 10, 2010 that the song's music video is her favorite one to date. She explained this was not only "because it was our first, but also because of the people involved. I just made it on a whim. We went down to the woods and we only had one camera. I got my dad to put a clown costume on and my friend's nephew to dress up as the baby clown while we decorated the woods. Dog walkers gave us the weirdest looks. It was really fun."
Before she'd ever recorded any music Welch met Isabella Summer, an aspiring hip hop DJ from the Suffolk seaside town of Aldeburgh. Summer had access to a studio in Crystal Palace, south London and the duo began working together, challenging each other to write a song in ten minutes, or make music that didn't involve guitar, bass or programmed beats. The first songs they co-wrote were this one and "Between Two Lungs."
Welch told MTV News about the song's music video: "I wanted it to be as artistic as possible. It wasn't about being pretty. I wanted something that was striking and made me look a bit scary."
The song featured during the trailer for Julia Roberts's Eat Pray Love movie and also in commercials for USA Network's spy drama Covert Affairs.
The song debuted on the Billboard Hot 100 dated September 25, 2010 after Florence + the Machine played the tune on the telecast of the VMA awards.
Welch recalled to Mojo how this became the breakthrough song she was looking for on her first album: "I was working with a lot of guitarists, and trying to record the beginnings of the first record. And nothing sounded right. It wasn't until I went, just for fun, to Isabella's (Summer's) studio, and we recorded 'Dog Days Are Over…' I did it without any guitar, and it was the first thing that I'd composed myself. There was no motive, or ego. And when I heard it without any guitar, and it was the right sound. It became this big kind of layered opus. And it sounded exciting. The label who were trying to sign me were very confused when they heard it. They said, 'what is this?'"