Trading may be quiet on Christmas Eve, but that hasn't stopped the S&P 500 from notching another milestone. The index inched up 0.2%, good enough to hit a new intraday high of just over 6,921 points.

U.S. stocks were mostly higher in Wednesday's final hour of trading, with the Nasdaq Composite and S&P 500 still on track for their biggest intraday comebacks in up to nearly two years. The S&P 500 ...