Two of the three main US stock market indices opened in the green on Monday, marking a positive start to the Christmas-shortened trading week.The tech-heavy Nasdaq Composite gained 0.51% to trade at 19,671.93.The broader index, S&P 500, surged as much as 0.23% to 5,944.60.The Dow Jones Industrial Average declined 0.26% to 42,730.30.