Homenoring the 21st night of September: SEPTEMBER - The story of the song begins in 1978.

Allee Willis was a struggling songwriter in Los Angeles, California, until the night she got a call from Maurice White, the leader of Earth, Wind & Fire. Maurice offered her the chance of a lifetime: to co-write the band's next album. Willis arrived at the studio the next day hoping it wasn't some kind of cosmic joke.

"As I open the door, they had just written the intro to 'September.' And I just thought, 'Dear God, let this be what they want me to write!' Cause it was obviously the happiest-sounding song in the world," Allee says.

Using a progression composed by Earth, Wind & Fire guitarist Al McKay, Maurice and Allee wrote the song over the course of a month, conjuring images of clear skies and dancing under the stars. Alee says she likes songs that tell stories, and that at a certain point, she feared the lyrics to 'September' were starting to sound simplistic. One nonsense phrase bugged her in particular.

"The, kind of, go-to phrase that Maurice used in every song he wrote was 'ba-dee-ya,' " she says. So right from the beginning he was singing, 'Ba-dee-ya, say, do you remember / Ba-dee-ya, dancing in September.' And I said, 'We are going to change 'ba-dee-ya' to real words, right?' "

Wrong. Allee says that at the final vocal session she got desperate and begged White to rewrite the part.

"And finally, when it was so obvious that he was not going to do it, I just said, 'What the f- - - does 'ba-dee-ya' mean?' And he essentially said, 'Who the f- - - cares?'" she says. "I learned my greatest lesson ever in songwriting from him, which was never let the lyric get in the way of the groove."

I asked Jeffrey Peretz, a professor of music theory at New York University's Clive Davis Institute, what makes that groove so powerful. He says a lot of it has to do with how the music unfolds. The song's very structure is an endless cycle that keeps us dancing and wanting more.

"There's four chords in the chorus that just keep moving forward and never seem to land anywhere — much like the four seasons," he says. "It's the end of summer, it's the beginning of fall, it's that Indian summertime, it's the transition from warm to cool."