Do you remember the 21st night of September?
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 fuck does 'ba-dee-ya' mean?' And he essentially said, 'Who the fuck cares?' I learned my greatest lesson ever in songwriting from him, which was never let the lyric get in the way of the groove." […]
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.