The mood in markets has flipped in the past hour and the S&P 500 is down 19 point at the open. It was trading higher up until 8:30 am ET.

It's quad witching today which means it's the expiring of monthly:

  • stock index futures,
  • stock index options,
  • stock options
  • and single stock futures

I tend to think the importance of it matters less with so much trading in weekly and daily options now but the lore continues. Perhaps more importantly, Brent Donnelly points out that Sept 16-Oct 9 is the worst season period of the year. I think some of that is a skew due to the Lehman year but it holds up nonetheless.