Why Trading Day 137 Marks the Start of the S&P 500’s Cruelest Season

August and September are the only two calendar months that tend to decline on average, writes SentimenTrader Senior Research Analyst Jay Kaeppel. Looking at the annual seasonal trend of the shows that the index often experiences a lull between the 137th trading day of the year and the 197th, he notes. In fact if you invested $1 in the S&P 500 index only from trading day #137 through trading #197 every year since 1985, you would wind up with about 55 cents.