Why Stocks Dipped in September but 2026 Is Still Up

The S&P 500, a basket of 500 large U.S. companies, was down 1.75% in September as of Sept. 20, 2026. The Nasdaq Composite was down 1.5% that month. Yahoo Finance

September is usually the weakest month for stocks. Since 1928, the S&P 500 has fallen 1.1% on average in September. MarketWatch Like a rainy-season average, it shows what often happens, not what must happen.

The drop came after a good year. The S&P 500 was up 12.3% in 2026 to that point, on pace for a fourth year of gains in a row. Morningstar

Daily moves were mixed. On Sept. 18, 2026, the S&P 500 rose 0.17%. Reuters On Sept. 2, 2025, it fell 44.72 points, or 0.7%, to end at 6,415.54.

Part of the action was about who is in the index. Bloom Energy, Illumina and Everpure were set to join the S&P 500 before markets opened on Monday, Sept. 21, 2026. S&P Global Funds that copy the index had to buy those stocks and sell others, which adds trading around that date.

August was already uneven. Palantir and Salesforce were among the best stocks that month in August 2026, while AppLovin and Honeywell were among the worst.

The broader context here is what that average means when the year is already strong. A 1.1% average loss does not promise a loss. Some Septembers go up. With the market up 12.3% and aiming for a fourth winning year, the choice for ordinary savers was whether to sit tight through September, try to protect against a dip, or hold less stock and risk trailing behind the index.

Looking at what this means for your money, this mix points to bumpy results for different stocks rather than all stocks falling together. Trading was focused around the Sept. 21 changes in a few names. When winners and losers split apart like that, your own result can look different from the index, based on what you own, how easy it is to trade around the switch, and whether you can handle lagging for a short time if September does not follow the usual pattern.