Amazing that young people even know the lyric. That’s an old as hell song.
Because it’s a timeless song. Per wiki:
“September” has a funk groove based on a four-measure pattern that is consistent between verses and choruses, built on a circle of fifths.
Using a chord progression written by Earth, Wind & Fire guitarist Al McKay, vocalist Maurice White and songwriter Allee Willis wrote the song over one month. Willis was initially bothered by the gibberish “ba-dee-ya” lyric White used through the song, and begged him to rewrite it: “I just said, ‘What the f— does ‘ba-dee-ya’ mean?’ And he essentially said, ‘Who the f— cares?’ I learned my greatest lesson ever in songwriting from him, which was never let the lyric get in the way of the groove.” The song was included on the band’s first compilation—The Best of Earth, Wind & Fire, Vol. 1—solely to boost sales with original content.
Although several theories about the significance of the date have been suggested, the songwriter Maurice White claimed he simply chose the 21st due to how it sounded when sung. His wife, Marilyn White, however, claimed that September 21 was the due date of their son, Kahbran, according to lyricist Allee Willis.