U.S. stock-market investors are staring down what has historically been the worst month of the year for Wall Street, but the strength equities have shown so far in 2018 suggests such weakness may not be repeated this year. According to Bespoke Investment Group, the seasonal weakness of the month of September is largely an issue of momentum. While the month averages negative returns overall, September tends to be positive in years where the market has been positive going into the month.via