Every year as summer ends, the same chart returns: average monthly S&P 500 performance since 1928, with September standing out as the weakest month.
News guide
The September Effect Explained: What the Stock Market Data Really Says
september effect5 min read
September has delivered the S&P 500's weakest average monthly return since 1928, but the result is not a reliable trading signal. Here is how to read the seasonality data.
5 min read
Practice trading with Finelo
Practice in a simulator, learn with bite-sized lessons, and build confidence before risking real money.
Want to learn more?
Practice in a simulator, learn with bite-sized lessons, and build confidence before risking real money.
Explore FineloExplore Finelo's 28-day challenges
Turn learning into a daily habit with guided challenge paths.
Educational note: This article is for educational purposes only and does not constitute financial, investment, legal, or tax advice. Finelo does not recommend any security, strategy, platform, or transaction. Investing and trading involve risk, including possible loss of principal. Historical performance does not predict future results.
What is the September Effect?
The September Effect is the name given to the stock market's historically weak performance during September. Depending on the exact period and whether returns include dividends, the S&P 500's average September return since 1928 is roughly negative 1% to negative 1.2%.
The observation is real. The conclusion often drawn from it — that investors should sell at the end of August and buy back later — is much less reliable. The gap between an accurate statistic and an actionable decision is the important part of the story.
Researchers and market commentators have proposed several explanations, including post-summer portfolio rebalancing, mutual-fund fiscal calendars, tax positioning, and investor expectations that become self-reinforcing. None of these explanations turns the pattern into a dependable forecast for a particular year.
The historical statistic is real
RBC Wealth Management calculated that the S&P 500 declined an average of 1.2% in September from 1928 through July 2025. It also found that September returns were negative 55% of the time, compared with 39% for all other months in that study.
Those figures show that September has been unusually weak. They do not show that a decline is inevitable. A 55% historical loss rate is only modestly different from a coin flip, and the average is affected by a small number of severe episodes. September 1929, 1931, 2001, and 2008 all included major market stress. Large losses in a few years can pull down a long-run average even when many individual Septembers are positive.
Practice trading with Finelo
Practice in a simulator, learn with bite-sized lessons, and build confidence before risking real money.
Why seasonality is not a trading signal
Recent history illustrates the problem. The S&P 500 fell about 5% in September 2023, then rose about 2% in September 2024 and 3.5% in September 2025. The Nasdaq gained nearly 6% in September 2025.
An investor who sold every August 31 would have avoided some declines but also missed strong months. Selling can create taxable gains and trading costs, and the investor must make a second correct decision about when to return. A rule with only a small historical edge can be overwhelmed by those frictions and by a few unexpected market days.
The September chart is also a useful lesson in financial-media literacy. Misleading charts do not need fake numbers. Real data can imply more certainty than the underlying distribution supports. Before acting on any claim that begins with “historically,” ask how often the pattern occurred, how widely outcomes varied, and whether a handful of extremes drove the average.
Why this matters to you
Averages can hide the distribution. A monthly average says little about the range of possible outcomes in the next 30 days. Frequency, median results, and worst cases add essential context.
Calendar-based selling has costs. Avoiding September requires two timing decisions and can add taxes or transaction costs. The seasonal pattern does not remove those hurdles.
Expected volatility is something to plan for, not automatically trade. A diversified portfolio and an appropriate time horizon can be more useful than trying to predict one month from a historical calendar effect.
For broader context, see how to understand the stock market as a beginner, how to build a diversified portfolio, and the 2026 CLARITY Act deadline.
Sources and further verification
Frequently asked questions
What is the September Effect?
The September Effect is the historical tendency for broad stock indexes, including the S&P 500, to perform worse in September than in other calendar months. Since 1928, the S&P 500's average September return has been about negative 1% to negative 1.2%, depending on the data window.
Does the stock market always fall in September?
No. RBC Wealth Management found that the S&P 500 had negative September returns 55% of the time through 2024. That is more often than in other months, but still far from a certainty. September 2024 and September 2025 both finished higher.
Should investors sell stocks before September?
A calendar average alone does not establish that selling is appropriate. Market timing requires both an exit and a re-entry decision and can create taxes, transaction costs, and missed gains. Investors should consider their time horizon, diversification, and risk capacity instead of treating seasonality as a rule.
September EffectS&P 500seasonalitymarket timinginvestingbehavioral finance
Practice trading with Finelo
Practice in a simulator, learn with bite-sized lessons, and build confidence before risking real money.
About the author
Finelo Team
The Finelo Team creates practical investing and trading education designed to help beginners learn faster with structured challenges, simulator practice, and bite-sized lessons.
Keep reading — Related articles
News
Nvidia–Hugging Face Deal: What the $12.9 Billion Agreement Changes
Nvidia agreed to acquire Hugging Face for $12.93 billion while promising to keep the platform open. Here is what the price, developer reach, and neutrality pledge mean.
Finelo Team
News
Lululemon Stock Falls to 2018 Levels After Its Q2 Earnings Report
Lululemon shares fell below $100 after revenue declined, comparable sales dropped, and management cut its outlook. The report offers a practical lesson about brand moats and falling knives.
Finelo Team
News
Why Diesel Prices Are Near Record Highs: The Crack Spread Explained
A closely watched U.S. diesel crack spread topped $106 per barrel while retail diesel approached its 2022 record. Here is how refining constraints can reach freight costs and consumers.
Finelo Team