TL;DR: Studies on Friday the 13th stock returns contradict each other. In the US, Kolb and Rodriguez (1987) reported significantly lower returns, but Dyl and Maberly (1988), using a longer series, found the effect vanished or even reversed. Lucey (2000) surveyed international markets and tied the question to the efficient market hypothesis. No study has examined whether Vietnamese lunar bad days (tam nuong, nguyet ky) affect the Vietnamese stock market. Even if a real effect existed, it would be very small, unstable, and must not be confused with a spiritual system "predicting correctly".
Short answer: yes or no?
There is no stable evidence that Friday the 13th makes stocks fall. Studies on the topic give conflicting results, and any effect, if it exists, is very small and does not replicate across different data series. This is a classic question in behavioral finance: if a belief is widespread enough to change trading behavior, does the price reflect that belief?
The three most cited studies are Kolb and Rodriguez (1987), Dyl and Maberly (1988), and Lucey (2000). The first two directly oppose each other: the later one rebuts the earlier with longer data. The third extends to many international markets. The results are not consistent, and that inconsistency itself is the answer.
What do studies on Friday the 13th say?
| Study | Market | Data period | Main result |
|---|---|---|---|
| Kolb and Rodriguez (1987) | US stocks | Friday the 13th returns significantly lower than other Fridays | |
| Dyl and Maberly (1988) | S&P 500 index | 1940-1987 | Friday the 13th returns not lower, even slightly higher than other Fridays; authors conclude the anomaly does not exist |
| Lucey (2000) | Multiple international stock markets | Examines Friday the 13th returns and discusses implications for the efficient market hypothesis |
According to Kolb and Rodriguez (1987), US stock returns on Friday the 13th were significantly lower than on other Fridays. This is a level B result: it was published in a peer-reviewed journal, but has not been stably replicated on other data series, and was rebutted shortly after.
Dyl and Maberly (1988) used the S&P 500 index for 1940-1987 and found Friday the 13th returns were not lower, even slightly higher than other Fridays. The two authors concluded the anomaly does not exist. This is a level A result within the scope of their data and method: the S&P 500, 1940-1987. It does not automatically hold for other markets.
Lucey (2000) examined Friday the 13th returns across many international stock markets and discussed the implications of the results for the efficient market hypothesis. This is level A within the markets the author surveyed. This article only cites the source's summary, adding no figures or sample sizes the source does not provide.
Why does this question matter for the efficient market hypothesis?
The efficient market hypothesis says prices already reflect all available information. If a calendar date, simply because it falls on the 13th and is a Friday, systematically produced lower average returns, that would be public, free information everyone knows but prices had not fully absorbed. An investor could short on Thursday and buy back on Friday to capture the difference. If everyone did so, the difference would vanish.
That is why this question is not just about superstition. It is a small, clean, easily testable probe of a large assumption in financial economics. Lucey (2000) framed the issue at exactly that level: what do Friday the 13th results mean for the philosophical foundations of financial economics.
When results are unstable, there are two readings. First: the effect is real but small enough to be drowned by other factors, so one study finds it and another does not. Second: the effect is not real, and the initial positive result was a product of data selection, period selection, or test selection. Both readings are consistent with the existing data. This article does not pick a side.
If belief changes behavior, can results change too?
There is a possibility between the two poles of "real" and "not real": the effect could arise from the belief itself. If enough people believe Friday the 13th is a bad day and actively sell or refrain from buying, that selling pressure could push prices down in the short term. In that case the effect is real, but it is not evidence that the day is inherently bad. It is evidence that belief changed behavior, and behavior changed outcomes.
In other words: if an effect appears, we must not call it a "spiritual system predicting correctly". And if no effect appears, we must not deny that the belief influences some people's behavior. These are two different things. This article discusses tests and data, not who is right or wrong in their private lives.
Have Vietnamese calendar bad days been tested?
No study in this article's source list examines the effect of lunar bad days (tam nuong, nguyet ky) on Vietnamese stock returns. This is a gap in the literature, not a result. It does not mean there is no effect; nor does it mean there is one. It simply means no one has published research this article could find.
In Vietnamese folk belief, certain lunar dates are considered bad days for starting ventures, signing contracts, or making large purchases. Concepts such as tam nuong and nguyet ky belong to this layer of belief: people avoid them, pass them on by word of mouth, and sometimes shift their business calendar accordingly. But whether that belief changes trading behavior in the stock market, and if so by how much, is a question with no data.
The lack of research in Vietnam does not allow extrapolation from foreign results. Studies on Friday the 13th were done on the US market and some international markets, with different investor structures, legal frameworks, and religious compositions. Results in the US do not automatically hold for Vietnam, and vice versa. A simple rule: results obtained in a given country, on a given group of people, speak only about that country and that group, until replicated elsewhere.
What beliefs are in the same family as numerical beliefs?
Friday the 13th is not the only belief testable against price data. In the same family is belief in lucky numbers in trading: some investors favor stock codes, numbers, or dates with digits that carry good meaning in their culture. The question is identical: if that belief is widespread and strong enough, does it leave a trace in returns? We discussed that question in detail in the article Do investors who favor the number 8 earn more?.
What these tests share is that they are testable, and results tend to be small, sensitive to sample selection, and unstable over time. That does not make the belief meaningless in the life of the believer. It only means that once placed on the scale of price data, we must accept that the scale may return a result different from what we hope.
How do you read research results correctly?
A "statistically significant" result does not mean "large" or "replicable". In finance, an effect that is statistically significant but amounts to a few thousandths of a percent per day is nearly unusable for trading after fees and slippage. And an effect seen in one period can vanish when the series is extended, exactly as in the case of Kolb and Rodriguez (1987) being rebutted by Dyl and Maberly (1988) with 1940-1987 data.
When reading an article about a calendar anomaly, ask four questions. Which market's data, which period? How is "Friday the 13th" defined and what is the comparison group? Was it tested across multiple markets and sub-periods? And does the result replicate in other independent studies? For Friday the 13th, the answer to the last question is: not yet, or it replicates in the opposite direction.
Conclusion: no stable effect, sources still debate
As of the studies in this list, there is no stable effect of Friday the 13th on stock returns. Kolb and Rodriguez (1987) once reported a negative effect, Dyl and Maberly (1988) rebutted it and found no effect on the S&P 500 for 1940-1987, and Lucey (2000) extended to many international markets and raised the issue at the level of the efficient market hypothesis. The sources still debate, and this article preserves that debate rather than picking a side.
Where evidence is missing, we say so clearly: no study in this list covers lunar bad days on the Vietnamese market. That is an open gap, not a conclusion. If someone wanted to test it, Vietnamese market price data is public, and the question is specific enough to answer with standard methods. Until such a result exists, the most honest thing to say is: unknown.
