Triple witching day combined with quarterly rebalancing, how will US stocks move next week?

According to PolyBeats monitoring, on Friday, September 18, the S&P 500 rose 12.74 points, with the index turning positive in late trading driven by the semiconductor sector; the Philadelphia Semiconductor Index rose about 2.78% that day. Currently on Polymarket, the probability of "S&P 500 closing higher on September 21" is 58%.

September 18 is also a quarterly "triple witching day." Triple witching refers to the simultaneous expiration of stock options, index options, and index futures on the same day, occurring on the third Friday of March, June, September, and December each year.

The biggest impact of triple witching is increased trading volume and mechanical capital flows, and it does not inherently signal bullish or bearish sentiment. On Friday, derivatives with a notional scale of about $7 trillion expired, and index option hedging, quarterly fund rebalancing, and closing auctions may amplify late-session volatility.

Historical research generally suggests that the next trading day after triple witching is more prone to a "late-session reversal," but it does not necessarily mean a decline or rise. Early market research found that if the index falls significantly in the last half hour of the expiration day, the Monday open often sees some degree of reverse repair; the opposite is also true. Institutions such as Citi have pointed out that triple witching usually brings higher trading volume, but not necessarily higher sustained volatility.

On Monday, Chicago Fed President Goolsbee will deliver a speech, and investors will focus on his remarks on inflation, employment, and the subsequent interest rate path. If the 10-year U.S. Treasury yield continues to stand above 5%, it may pressure high-valuation tech stocks; if yields fall back, the semiconductor sector may have an opportunity to continue supporting the index. The U.S. manufacturing and services PMI released midweek will also affect the market's judgment on the economy and interest rates in advance.