The stock market history of the world is full of episodes that turned autumn into a synonym for risk: 1929, 1987, 1997, 2001, and 2008 are a few examples of years that left behind some of the most violent trading days and periods in these markets.
September is, statistically, the worst month for U.S. equities, while October concentrates several of the largest crashes in history. The above is relevant because in 2026, the calendar once again finds a highly valued market, concentrated in technology and facing fresh pressures on interest rates, inflation, and geopolitics. Although nothing is written and no one can predict the future, it is always important to remember the lessons of history.
It is a fact: Wall Street is about to enter one of the times of the year that instills the most respect among investors; anything can happen.
Not because September or October have, in themselves, the power to cause a crisis. Financial history does not work according to calendars. But a hard-to-ignore coincidence exists: some of the largest episodes of stock market wealth destruction in the modern era occurred during these two months.
The most famous precedent is October 1929. But then came October 1987, October 1997, September 2001, and the dramatic September–October period of 2008. They are different episodes, caused by different problems, but all left the same lesson for asset managers: when a market reaches a zone of high confidence, leverage, concentration, or valuation, any catalyst can turn a correction into a crisis.
September is not superstition, it is the worst month on the calendar; October is the month of frights
The data partially supports the month’s bad reputation. The S&P 500 has historically recorded a negative average return close to -1% in September, making it the month with the worst average performance of the year. Data from S&P Dow Jones Indices show that, since 1928, September records an average return of approximately -1.03% and finishes with gains only about 44.7% of the time.
More recent data point in the same direction. For the long period analyzed by Dow Jones Market Data, both the S&P 500 and the Dow Jones Industrial Average lose an average of around 1.1% in September, while the Nasdaq Composite records an average drop close to 0.8%.
The statistics do not mean that September will be negative every year. In fact, the market can rise strongly during the month. What they mean is that, statistically, the distribution of outcomes is less favorable than in other months. And here appears the first important difference between September and October.
October has a worse reputation, but September is usually worse in terms of average return. October is, above all, the month of big frights. Cboe has noted that October has historically displayed the highest levels of monthly volatility for the S&P 500, although a large part of that characteristic is influenced by extraordinary episodes such as 1987 and 2008.
In other words: September tends to penalize performance more; October has a stronger historical association with extreme moves.
1929: The autumn that forever changed financial history
The first major chapter began even before October. During the 1920s, speculation drove the Dow Jones Industrial Average from 63 points in August 1921 to 381 in September 1929—an increase of approximately six times in eight years. The market reached levels that seemed to justify the idea that it had entered a new era of permanent prosperity.
But the reality was very different; after the September peak, signs of deterioration began. On October 28, 1929, the so-called Black Monday, the Dow lost nearly 13%. A day later, Black Tuesday, it plunged another 12%. By mid-November, the Dow had lost virtually half its value from its peak.
However, the real impact was much greater than the stock market crash; the collapse damaged bank and corporate balance sheets, caused credit contraction, and ended up becoming part of the process that led to the historic Great Depression, the worst U.S. economic contraction of the 20th century, which lasted from 1929 to 1941.
A historical clarification is important: 1929 did not cause the Great Depression on its own. The crash was the financial trigger of a much broader process involving monetary contraction, banking failures, deflation, falling international trade, and other factors.
1987: When the Dow lost 22.6% in a single day
Nearly six decades later, October again became synonymous with panic. It was October 19, 1987—Black Monday—when the Dow Jones plunged 508.32 points, equivalent to 22.61%, the largest single-day percentage drop in its history.
The figure remains impressive: the drop far surpassed the record of 12.8% set on October 28, 1929. The destruction of wealth was devastating; over $500 billion in market capitalization vanished from the New York Stock Exchange that day, while 604.33 million shares were traded, approximately three times the daily average at the time.
Yet 1987 left another fundamental lesson: a market can suffer an extraordinary crash without necessarily triggering an economic depression. The Federal Reserve reacted by providing liquidity to the financial system, and markets subsequently began to stabilize.
It also gave birth to one of the tools that forms part of today’s market infrastructure: circuit breakers, mechanisms designed to temporarily halt trading when declines reach specific thresholds.
1997: The Asian crisis reaches Wall Street
Ten years later, October proved once more that a crisis can travel quickly across regions due to a new era: the era of globalization. On October 27, 1997, amid the Asian financial crisis, the Dow lost 554.26 points, equivalent to 7.2%, after a plunge in Asian stock markets heightened fears regarding global growth and U.S. corporate earnings.
The episode was particularly relevant to the evolution of financial infrastructure; for the first time since their creation, circuit breakers were triggered on Wall Street. The market had to halt trading temporarily and close earlier than usual. The day proved that financial globalization had altered a core market feature: a localized shock could be transmitted to other continents in a matter of hours.
2001: September and the return of fear
The next major historical episode occurred precisely in September; after the September 11 terrorist attacks, U.S. markets remained closed for four trading sessions. When Wall Street reopened on September 17, 2001, the Dow Jones lost approximately 7%, while the S&P 500 fell around 5% and the Nasdaq close to 6.8%. The Dow lost nearly 679 points during the session, its largest single-day point drop at that time.
It was not merely an emotional reaction. The market was already weakened by the bursting of the tech bubble and a deteriorating U.S. economy; September simply concentrated the shock.
2008: When September stopped being a month and became a crisis
The most relevant episode for today’s investors may be 2008; on September 15, 2008, Lehman Brothers filed for bankruptcy protection. The Federal Reserve has described that moment as a turning point that triggered a massive retreat of investors from risky assets and a loss of liquidity in short-term funding markets.
But the crisis did not end with Lehman; Fannie Mae and Freddie Mac had been placed under government conservatorship, AIG faced a liquidity crisis, and the money market fund industry experienced heavy withdrawals after a fund broke the $1.00 net asset value barrier.
During September and October, massive sell-offs spread across virtually the entire financial system; October 2008 ended with a monthly decline for the S&P 500 of nearly 16.9%, ranking among the worst months in the index’s history. The market was no longer reacting simply to bad corporate news; it was pricing in the possibility of a systemic credit crisis.
That is perhaps the main difference between a stock market crash and a financial crisis: the former destroys market value; the latter can simultaneously paralyze credit, the banking system, and the real economy.
The pattern exists, but it is not a prophecy
For portfolio managers, the most important conclusion is probably also the least spectacular: September and October carry no financial curse.
In fact, October finishes with positive returns more often than its reputation suggests. Between 1950 and 2024, the S&P 500 ended October with gains approximately 59% of the time, with an average return close to 0.85%. The issue lies in the magnitude of the extreme episodes.
October includes three of the worst months in S&P 500 history: October 1987 (-21.8%), October 1929 (-19.9%), and October 2008 (-16.9%). That is to say, October does not necessarily drop more than other months; it simply possesses an extraordinary capacity to feature in history books when things go wrong.
Therefore, using the calendar as an automatic sell signal would be a mistake. But using it as a reminder to review risks can be a rational decision, especially when entering autumn with its own set of vulnerabilities—which is when historical comparisons acquire relevance.
Wall Street closed August with gains: the Dow gained around 2.1% during the month, the S&P 500 3%, and the Nasdaq 4.1%, according to recent data. But behind that strength lies a market that is particularly sensitive to expectations surrounding artificial intelligence, interest rates, and growth; at the same time, the macroeconomic landscape has grown complicated.
On another note, no less relevant, military tensions between the United States and Iran pushed oil prices above $90 per barrel at times, while global bond yields rose and the market began pricing in a higher probability of a rate hike by the Federal Reserve in September.
The FOMC meeting is scheduled for September 15 and 16, meaning the market will enter the month with one of its main catalysts occurring right within the historically weakest period for equities—a combination that is especially relevant for asset managers.
A market concentrated in a handful of technology companies can appear solid as long as investors continue paying elevated multiples for future growth. However, if expectations regarding interest rates, inflation, growth, or the return on artificial intelligence investments shift simultaneously, the same concentration that propels the market during rally phases can amplify losses during a correction.
Added to this is the growth of leveraged strategies and derivative products. In 2026, for example, the number of single-stock ETFs with leveraged or inverse positions has multiplied extraordinarily, accelerating the speed at which specific moves can transmit across equities and derivative products.
The real lesson for funds and wealth management
For fund managers, family offices, and wealth managers, the lesson of September and October is not to exit the market, but to ask what would happen to a portfolio if the scenario changes rapidly.
Historical lessons serve precisely that purpose: 1929 taught the danger of leverage and speculative bubbles; 1987 showed that automated trading mechanisms can amplify extreme moves and that market liquidity can vanish much faster than anticipated; 1997 confirmed that financial shocks travel globally; 2001 showed how a geopolitical shock can hit an already weakened market; and 2008 left perhaps the most important lesson for institutional investors: the real risk lies not just in falling stock prices, but in the simultaneous disappearance of liquidity across multiple markets.
That is why, as September 2026 begins, history is not saying that Wall Street will necessarily fall, but it is saying something far more useful for a professional investor: when valuations are elevated, positions are concentrated, and the cost of money becomes a market variable once again, it pays to enter autumn asking not how much higher a portfolio can go, but how much it could lose if history decides to repeat itself.
Because September does not cause crashes, but history proves that when a market enters autumn feeling vulnerable, September and October have proven capable of turning a crack into a fracture.
