The following chart shows three different calculations of the S&P 500's seasonality [...] one commonality is the V-shape into early October and out of it. That is the reason why everyone is talking about the weakness in September.
The following chart shows the seasonality of all the years since 1950. The seasonality only touches the low of the chart once, which means that over all those years, the market has always had one weakness at the same time. The low happens in late September, followed by a substantial rally until January. The seasonality shows some weakness into March, followed by a reaction that ends at the beginning of May. After that, the seasonality, calculated without a trend component in this chart, goes down until late September. This is why we often say, “Sell in May and go away.”
Summary:
While all seasonalities point to a weak September and a crucial low late in September, the seasonality of the years ending in 4 lets us assume that the seasonal low could occur one or two weeks later than usual. Anyway, no other date in a year is statistically as significant for the S&P 500 as everyone knows and is waiting for the late September or early October low.
