21 Septemba bu ubochi nke 21st (nke iri abụọ na out) na onwa ana kpo Septemba. Na Gregorian calender, 21 Septemba bu ubochi nke 264th (265th na afo nke ana-kpo leap year) na afo. Anyi nwere agugu ubochi 101 du mgbe afo na agwu.