The month of September is historically the worst for investor returns. The S&P 500 over the past four years has averaged a 5.8% decline in September. However, it’s not just the S&P 500 that does poorly; so does the Dow Industrials, the NASDAQ, and the Russell 2000. “While momentum usually overrides seasonal concerns, September is hard to ignore – it is the only month down on average,” CNBC reported comments from Frank Gretz of Wellington Shields in an Aug. 30 note to clients. As I write this, all four indexes are down over 1% in early Tuesday morning trading. History appears to be ready to rep…