Fall Seasonality Weighs On S&P 500

Across multi decade data, September and October stand out as the weakest seasonal window for the S&P 500 (SPX), with the index historically delivering lower average returns than in most other months. Studies over roughly 1950s to 2020s also show August–October as the softest three month stretch on average.

Technical work using moving average metrics indicates that in late summer and early autumn the S&P 500 (SPX) tends to spend more days below short term trend gauges such as the 20 and 50 day moving averages. During major autumn selloffs like 1987, 1997, 2001 and 2008, the index typically stayed decisively below its 50 day moving average for one to three months.

Because SPDR S&P 500 ETF Trust (SPY), iShares Core S&P 500 ETF (IVV) and Vanguard S&P 500 ETF (VOO) all track the index, this seasonal softness translates directly into their price behavior and their distance from the 50 day moving average. Equal weight exposure via Invesco S&P 500 Equal Weight ETF (RSP) reflects the same broad pattern when weakness is market wide rather than concentrated in mega caps.

Systematic strategies that reduce exposure when the S&P 500 (SPX) trades below its 50 day moving average can therefore concentrate de risking activity in the September–October window if the long run seasonality persists. The pattern remains conditional rather than guaranteed in any single year, but the clustering of weak returns and notable crash episodes in early autumn has made this period a recurring focus for S&P 500 linked vehicles and U.S. large cap equity indices tied to the benchmark.