September has been historically considered to be the worst trading month of the year on Wall Street. So much so that, since 1925, September has been the only month to return negative, per Fisher Investments data.There are various reasons behind this phenomenon. September ushers in the end of the summer vacations, and traders rebalance portfolios, waking up to new market realities. This increases trading volume, and stock prices feel the heat. People rush to the safety of bond markets and invest money they w ...