Every October, many stock market investors celebrate their own pre-Halloween scare tradition, thanks to a calendar-based phenomenon called the “October Effect”, which is “the theory that stocks tend to decline during the month of October.”

As Investopedia goes on to describe it, “the October effect is considered mainly to be a psychological expectation rather than an actual phenomenon. Most statistics go against the theory.” And indeed, that’s true. Examining the month’s track record over the years, one finds that truly scary market events like the Black Monday Crash of 1987, which celebrated its 30th anniversary this week, are often balanced against the month’s coincidental timing in marking the beginning of long term rallies.

Still, we can still offer some insight into why both those phenomenons exist in the month of October. It is because the second week of the month marks the beginning of the year’s fourth quarter’s earnings season, which is the time that publicly-traded U.S. firms will announce if they are doing better or worse than expected for the year, where company leaders will also communicate changes in their expectations of how their businesses will perform in the next year.

ZeroHedge is reporting an example of the negative side of this dynamic play out on this Friday, 20 October 2017, with General Electric (NYSE: GE), which for GE investors, looks like a Black Friday-type event.