One of the key features of the 2018 edition of the stock market is that it has remained surprisingly durable in the wake of heightened tensions from President Donald Trump’s continuing trade war and the Federal Reserve’s shift to a more hawkish stance. Now, the market will be put to an even more rigorous stress test in September, with the month among the worst for stock performance. Since its inception in 1928, the S&P 500 SPX, +0.01% has finished lower in September more than any other month, or 55% of the time for an average return of negative 1.01%, according to Dow Jones Data Group. For the Dow Jones Industrial Average DJIA, -0.09% the record is equally dismal, with the blue-chip benchmark also seeing its worst performance in the month, declining 70 times out of 121 Septembers for an average return of negative 1.03%. September is likewise a poor month for the Nasdaq COMP, +0.26% with the technology-heavy index falling 45% of the time since 1971 for a negative return of 0.49%. Stock returns for each month of the year via