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Stock market seasonality describes historical differences in returns across parts of the calendar, including the “Sell in May and Go Away” pattern. Those patterns have appeared in some markets and samples, but they do not reliably predict what stocks will do in a particular market or year. Before changing a portfolio based on the calendar, consider the evidence alongside costs, taxes, time horizon, risk tolerance, and financial goals.
What does “Sell in May and Go Away” mean?
Also called the Halloween indicator, “Sell in May and Go Away” is a hypothesis that stock returns have tended to be higher from November through April than from May through October. It compares the two halves of the year; it does not mean stocks necessarily fall every summer.
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The January effect is another subject in the seasonal-anomaly literature. Its recognition as a research topic does not establish January as the best month for every investor to buy stocks. Monthly averages describe past samples, not a dependable forecast.
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteWhat have studies found across markets?
Results vary with the market and the sample. Two studies illustrate why the evidence should be read in context rather than treated as a universal trading rule.
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| Study | Reported evidence | What the figures mean |
|---|---|---|
| Tomasz Schabek and Henrique Castro, 2016 | The Halloween effect was statistically significant in 19 of 73 markets, including 11 of 23 markets with long time series. | These are counts of markets in the study, not the odds that a seasonal strategy will succeed in the future. The authors also report that the effect persisted after controls for selected weather, behavioral, and macroeconomic factors. |
| Ben Jacobsen and Cherry Yi Zhang, 2021 | The study description reports 62,962 observations across available stock-market indices, covering 114 countries for market price returns and 65 markets for total returns and risk premia. | These figures describe the breadth of the historical dataset, not a guaranteed result for a particular country, investor, or implementable strategy. |
A large sample or a statistically significant historical average can help identify a pattern worth studying. Neither establishes that the pattern will continue or remain useful after implementation costs.
How can you judge a seasonal-return claim?
Two claims about seasonality are only meaningfully comparable when their definitions and methods line up. Check these details before treating an historical finding as an investing strategy:
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- Market or index: Identify the country and index studied; results from one market need not apply to another.
- Sample dates and length: Find the start and end dates and how much history the result covers.
- Return measure: Distinguish price returns from total returns, which include reinvested distributions.
- Statistical support: Look for significance tests and robustness checks, and note whether the result holds across markets or only a subset.
- Real-world implementation: Ask whether the proposed strategy accounts for trading costs, fees, and taxes. A historical pattern and a net-of-costs strategy are different claims.
What can go wrong when you try to time the market?
Trading costs and fees
Switching in and out of investments can mean more trades and higher transaction costs or fees, which reduce returns.
Missing a rebound
Some strong market days occur during volatile periods. An investor who sells during a temporary decline may be out of the market when it recovers.
Tax consequences
Selling can realize gains. FINRA notes that, in the United States, holdings kept for less than a year may be subject to higher short-term capital-gains tax rates. Actual tax treatment depends on individual circumstances; this is not individualized tax advice.
Mismatch with your portfolio
A calendar-based switch may conflict with an allocation chosen for your time horizon and risk tolerance. Investor.gov says allocation decisions depend on those personal factors. Diversification may reduce the impact of a loss in one investment, but it cannot guarantee protection when the market falls.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should you consider before changing your investments?
- Start with your plan: Assess your time horizon, risk tolerance, and financial goals before considering a seasonal adjustment.
- Check the evidence: Confirm which market, dates, and return measure support the pattern, and whether the claim accounts for implementation costs.
- Estimate the consequences: Consider the costs of extra trades, possible taxes on sales, and the risk of being out of the market during a rebound.
- Choose a process you can follow: FINRA identifies buy-and-hold and periodic investing as alternatives to active market timing. Dollar-cost averaging means investing equal portions at regular intervals regardless of market ups and downs; it provides a consistent process, not a promise of profit.
In its June 10, 2025 article “What Is Market Timing?”, FINRA cautions: “Don’t let short-term emotions about investments disrupt your long-term financial objectives.” A seasonal pattern should be weighed against that broader objective, not treated as a substitute for it.
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