August the best month for average stock market performance? Or is it the worst? The answer depends on the period of stock-market history you examine. Over the 90 years from the Dow Jones Industrial Average’s
inception in 1896 until 1986, August on average was far ahead of the other months — more than four times larger, as you can see from the table below. August outperformed the other months’ average by 1.4 percentage points. This difference is significant at the 95% confidence level that statisticians often use when determining if a pattern is genuine.
In the years since then, in contrast, August has been the worst month for the stock market, on average, lagging the other months’ average by 1.7 percentage points. Since 1986, in fact, August has been a worse month for the stock market than even September, whose reputation for stock market losses is widely known.
August’s average DJIA return
Average return of all other months
August’s rank among all 12 months
1896 to 1986
If the 36 years since 1986 were …