Since Sept. 13 (Elul 29) falls on a Sunday, the last day the markets are open will be.....September 11, the same day as the attack on the World Trade Center, and the beginning of our largest recession, and cause of our largest point drop in history at that time.
Sept 13th also hosts the second of noteworthy eclipses for 2015. The last time a solar eclipse happened on Elul 29?--Black Monday, 1987.
Too many things happening on that day to ignore.
But I generally default to that verse that says "no one know the date, not even the Son of Man."