SPY seasonality chart of the last 30-years, via SentimenTrader. Click to enlarge charts.
SPY September stats dating back to 1993, courtesy of PastStat.
Seasonal Chart of the Dow Jones in election years, via @heynow
Here's another great chart via AlmanacTrader that shows the seasonal pattern in eighth years.
We also have a ton of other evidence that September might be a rough month for the bulls; Hedgefund exposure at nosebleed levels, VIX seasonality strong in September, a record number in VIX shorts, record long in SP500 futures by speculators, etc.
All the above facts are irrelevant until we start to see some price deterioration in the SP500.
The first step would be for it to trade below its 5-day moving average and then have that average act as resistance.
Secondly, we need to see some weakness in some of the momentum names, specifically ACIA, TWLO, and SMH. Until then it's going to be hard to press shorts.
My opinion and outlook are subject to change as new information comes in.