As Rob Hanna posted at Quantifiable Edges, this week has historically been the worst week of the year for US stock performance. Since 1960 the week following the 3rd Friday in September has produced the most bearish results. The chart below shows how this week has played out over the past 59 years.
As you can see there was a stretch back in the 80’s with a series of mild up years but overall, and especially since 1990 its been pretty much all downhill. The table below shows the result of buying the SP500 on the 3rd Friday of September (options expiration) and selling it x-days later (x days = 1-5 reflecting the entire week)
On of the past 28 years, only two, 2001 and 2017, posted a gain. 2001, of course, provided an uncommon backdrop and 2017, saw the index rise a whopping 2 points.
Unless you are a short-term trader this information does not provide an investment edge. For those with longer time frames, this data should be viewed as nothing more than setting an expectation for what may be a disappointing week for those invested in US stocks.