Stocks turned in uneven gains early Monday as the short holiday week got off to a wobbly start. The Nasdaq put in a firm 0.5% gain. The S&P 500 was steady, up 0.2%. The Dow Jones industrial average trimmed its early advance to less than 0.1%. Volume was down sharply: off 45% on the Nasdaq and 55% lower on the NYSE compared with Friday’s options expiration-fired trade. The stock market today opened a week of busy economic news on a down note.