If you've spent any time around markets, you've heard the sayings: "Sell in May and go away." "Watch for the Santa Claus rally." These are examples of seasonality — the idea that markets tend to behave differently at different times of the year. Seasonality is one of the oldest observations in trading, and it's easy to misuse. This guide explains what it is, what the historical data really says, and how to fold seasonal patterns into your research without treating them as a crystal ball.
What is seasonality?
Seasonality is the tendency of a market to show a regular, repeating pattern tied to the calendar. That can play out across days of the week, months of the year, six-month stretches, or even multi-year cycles. The core idea is simple: instead of asking only "what is price doing now?", seasonality asks "what has this market typically done at this time of year, averaged over many years?"
A seasonality chart answers that question visually. It takes years of history and plots the average path of a stock, index, or commodity across the calendar, so you can see whether a market has tended to rise into December, sag in September, or firm up in spring. It's a study of tendencies, not certainties.
Why patterns show up at all
Some seasonal effects have plausible real-world drivers. Holiday shopping in November and December can lift retail names. Summer travel season affects airlines, hotels, and energy demand. The back-to-school stretch in late summer touches consumer goods. Tax deadlines, fund reporting periods, and year-end positioning by big institutions can nudge whole indexes. When a driver is real and recurring, a seasonal pattern is more believable. When it's just a coincidence in the data, it tends to fade.
What the long-run numbers actually show
Two of the most-cited patterns are worth putting real figures against.
"Sell in May and go away"
This adage says the market does most of its work in the colder half of the year. The historical averages are striking: since 1950, the S&P 500 has averaged roughly 7.1% from November through April, versus only about 1.7% from May through October. Later data through 2024 tells a similar story — the "winter" half has meaningfully outpaced the "summer" half on average.
But averages hide a lot. The weaker May–October window still contains some of the market's largest single up days, and many of the strongest days cluster around bouts of volatility. An investor who mechanically sold every May would have sat out plenty of gains. "Sell in May" describes an average tilt — it is not a rule that pays off every year.
The Santa Claus rally
The "Santa Claus rally" refers to the tendency for stocks to rise over the last five trading days of December and the first two of January. On average, the S&P 500 has gained about 1.3% during that seven-day window, versus roughly 0.2% for an average seven-day stretch, and some studies find the rally has appeared in about 80% of years over the past five decades. That's a real, well-documented tilt — but "80% of the time" also means it has failed to show up in roughly one year out of five, and a missing rally has sometimes preceded weaker markets.
September, the perennial laggard
On the other end, September has historically been the weakest month for U.S. stocks, posting negative average returns across many decades, while November has often been among the strongest. Again: average is the key word.
How to actually use seasonality
Seasonality is a context tool, not a trigger. Used well, it can:
- Frame expectations. Knowing a market has historically firmed into year-end helps you interpret a rally as ordinary rather than surprising.
- Confirm, not initiate. Many traders look for seasonal tailwinds and a technical or fundamental setup pointing the same way, rather than acting on the calendar alone.
- Narrow a watchlist. Seasonal strength in a sector can be a reason to look closer — the starting point of research, not the conclusion.
Used badly, it becomes superstition — "it's November, so it has to go up." It doesn't have to do anything.
Reading a seasonal chart without fooling yourself
A few habits keep seasonality honest:
- Check the sample size. A pattern built on 40+ years of data is sturdier than one drawn from the last five.
- Look at the spread, not just the average. If the "average" December is up 2% but individual Decembers range from −10% to +12%, the average is thin comfort.
- Remember the regime can change. Structural shifts — new market participants, algorithmic trading, changed tax rules — can weaken a once-reliable pattern.
- Never size a position on seasonality alone. Standard risk practice (small position sizes, defined risk) matters more than any calendar edge.
You can explore historical seasonal tendencies across markets with TEG's Seasonality tool, which plots average price behavior over multiple years so you can judge a pattern's strength for yourself instead of relying on a saying.
The bottom line
Seasonality is a genuinely useful lens: markets do show repeating calendar tendencies, and the long-run numbers behind "Sell in May" and the Santa Claus rally are real. But every one of these patterns is an average stitched together from wildly different individual years. Treat seasonality as one input among many — a way to frame the odds, not to predict the outcome — and pair it with your own analysis and disciplined risk management.
Not financial advice. TEG's tools and articles are for education and research only. Markets carry risk, past performance is not indicative of future results, and nothing here is a recommendation to buy or sell any security. Do your own research and consider speaking with a licensed professional.
Sources: Britannica Money ("Sell in May and Go Away"); Financial Planning Association and Wikipedia (Santa Claus rally statistics); StockCharts ChartSchool (seasonality charts); Liberated Stock Trader and Trade That Swing (monthly seasonal patterns).