The coming week will be a massive event for earnings, with more than 150 Index companies set to report — including such behemoths as Apple and Exxon Mobil.

Yet for some traders, the biggest worry isn't the volatility that could be stirred by the reports, but rather a fact about the market's historical performance.

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Using data going back to the creation of the S&P 500 in 1957, technical analyst John Kosar of Asbury Research found that "the fourth week of October, which is next week, is seasonally the weakest of the entire fourth quarter."

When asked last week on CNBC's "Futures Now" what explains the trend, Kosar demurred.

"When you're looking at the fourth quarter, you look for things that stick out," he said. "You don't necessarily know why, but this is kind of one of those things."

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Still, not everyone is getting nervous.

"Just to bet on something without any fundamental justification behind it? I'd rather rely on studying chicken entrails or my Ouija board," said Chicago-based trader Scott Nations.

In fact, any trade based on the factoid would have to be an especially quick one. After next week, December — the strongest month of the year per Kosar — isn't far behind. During that month, the S&P tends to rise by 1.5 percent on average.

"So look for possible lows to be made next week and then after that in terms of seasonality, it looks like it's going to be blue sky," the technician said.

—By CNBC's Alex Rosenberg.

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