September has earned its reputation as the market’s least favorite month, and 2026 is shaping up to be an especially spicy edition. Barclays is telling equity investors to trim risk heading into fall, citing a cocktail of seasonal weakness, midterm election jitters, interest-rate volatility, and a wave of blockbuster AI IPOs that could test how much capital markets can absorb at once.
September’s track record speaks for itself
Since 1928, the S&P 500 has posted an average September return of roughly -1.1%. That makes it the only month on the calendar with a negative long-run average. The numbers get worse in midterm election years. During those cycles, the S&P 500 has historically declined about -1.5% in September. 2026 happens to be a US midterm election year, and the August-through-October stretch in such years has produced mixed results at best.
Barclays isn’t the only firm sounding the alarm. Citadel Securities has also reportedly advised reducing exposure or adding portfolio protection ahead of September.
AI IPOs could absorb a lot of oxygen
