The two months with the worst historical track record for stocks are September and August. Looking at long-term performance data for major indexes like the S&P 500 and the Dow Jones Industrial Average, September has consistently stood out as the single weakest month on average, while August has frequently ranked as the second-worst or otherwise underperformed most other months of the year.
September's poor reputation is not a recent phenomenon. Traders and analysts have tracked this pattern for generations, and it shows up across different decades, market cycles, and economic conditions. While no single explanation fully accounts for it, several theories are commonly cited. One is that institutional investors and fund managers often rebalance portfolios after summer, selling off underperforming positions before the end of the third quarter. Another is that trading volume tends to be lower during late summer, which can make price swings more pronounced when volume returns in September. Some also point to psychological factors, such as investors returning from vacation and reassessing their holdings with fresh eyes, sometimes leading to selling pressure.
August's struggles are often linked to thin trading volume as well. With many institutional investors, analysts, and executives on vacation, trading desks are frequently staffed at reduced capacity. Lower liquidity can mean that even modest selling triggers larger price moves than would occur during busier months. August has also been the backdrop for several significant market disruptions over the years, including currency crises and sudden volatility spikes, which may have reinforced its reputation even though not every August has been negative.
It's worth stressing that these patterns are based on historical averages, not guarantees. In any given year, September or August could easily post gains, and plenty of them have. Seasonal patterns like this are sometimes referred to as calendar effects, and while they show up with enough regularity to attract attention from researchers and traders, they are far from reliable predictors of what will happen in a specific year. Markets are influenced by countless factors, including interest rate decisions, corporate earnings, geopolitical events, and economic data, all of which can easily overwhelm any seasonal tendency.
Investors should also be cautious about reading too much into month-by-month averages when making real decisions. Trying to time the market by selling before a historically weak month and buying back in afterward is a strategy that sounds simple but is difficult to execute consistently. Transaction costs, tax implications, and the risk of missing a rebound can erode any theoretical benefit. Many financial advisors argue that staying invested through seasonal fluctuations, rather than attempting to dodge them, has historically been a more effective approach for long-term investors.
Other months have occasionally been cited in various analyses as weak performers too, including February, which some data sets show trailing the broader annual average. However, September and August are the two months most consistently flagged across different time periods and data sources as the weakest for stock returns.
For everyday investors, the takeaway is less about avoiding specific calendar months and more about understanding that volatility is a normal part of investing. Seasonal patterns can be interesting context, but they are not a substitute for a diversified portfolio and a long-term strategy suited to individual financial goals and risk tolerance.