did you remember the 21st night of september

The "21st night of September" in the song "September" by Earth, Wind & Fire has no particular significance beyond the fact that it simply sounded better than other dates when sung. According to Allee Willis, the Grammy-winning songwriter who worked on the song, they tried various dates during the writing process, and the 21st was chosen because it fit best lyrically and musically. It is not related to any historical event, personal memory, or celebration of the end of summer.