Stocks opened the final session of what has been a strong month on a weak note, and under a hail of middling economic news. The Nasdaq, the Dow Jones industrial average and the S&P 500 slipped 0.1% apiece. Volume was quietly mixed, up 1% on the NYSE and down 2% on the Nasdaq compared with trade at the same time Thursday. The stock market today received little help from the economic calendar.