We may want to pay close attention to the Stock Market today as September has been tough for stock investors. But if history is any guide, the last day of September may deliver one more blow to already battered markets, according to the financial blog Bespoke. Looking at data as far as 1945, the S&P 500 has posted positive returns just 38% on the last day of September, making it one of the worst trading days of the year, according to Bespoke (as the included table illustrates).

Earlier this month, financial blogger Ryan Detrick pointed out that the 38th, 39th and the 40th weeks of the calendar—which fall in September—tend to be the weakest of the year dating back to 1950. September has marked a particularly rough stretch for the S&P 500 SPX, +0.12% with only the week of Sept. 11 closing higher as China’s slowdown, global economic uncertainties, and lack of clarity on the timing of the Federal Reserve’s expected interest-rate hike have shaken investor confidence. According to FactSet, weekly performance in 2015 for the S&P 500 was among the worst in September. FULL REPORT