Mark Hulbert: The Dow usually slumps in September. The reason why is one of the market’s unsolved mysteries. Posted on August 23, 2022 By Charlielikes September is the worst month of the calendar for the U.S. stock market. Stock Market Tags:'is, ‘why, dow, hulbert:, in, mark’, market, markets, mysteries., of, one, reason, september, slumps, stock, stock market, the, unsolved, usually