U.S. stock market futures saw a muted start to New Year's Eve trading on December 31, with S&P 500, Nasdaq, and Dow futures all down slightly. Despite minor dips, the S&P 500 is on track for a significant annual gain of over 17%, marking its longest monthly winning streak since 2017. Thin trading volumes are expected to heighten market volatility, though some investors anticipate a 'Santa Claus rally' in early January.