S&P 500 and Nasdaq post worst week since June

U.S. stocks finished sharply lower on Oct. 16, 2026, according to MarketWatch. The S&P 500 — the broad index tracking 500 large U.S. companies — and the tech-heavy Nasdaq Composite posted their worst weekly performances since June. For savers in 401(k)s, that kind of slide cuts into retirement balances.