Stocks will likely continue adding to their gains, at least until the end of the year, as investors get more confident that the market's rally is sustainable, said Joe Bell, a senior equities analyst at Schaeffer's Investment Research.
"We've had a pretty decent run here," Bell said. "It wouldn't surprise me if we saw the momentum slow a bit through the end of October and then have a nice rally through November and December."
The S&P 500 rose three points, or 0.2 percent, to 1,747 in the first hour of trading. It's up 22 percent so far this year.
The Dow Jones industrial average edged down 17 points, or 0.1 percent, to 15,382. The Nasdaq composite rose eight points, or 0.2 percent, to 3,922.
The results were mixed among companies reporting earnings on Monday.
Hasbro surged after reporting that its net income rose 17 percent as sales increased. Its adjusted results and revenue topped analysts' estimates. The stock climbed $4.11, or 9 percent, to $51.35.
McDonald's fell 75 cents, or 1 percent, to $94.42 after the world's biggest hamburger chain's revenue fell short of Wall Street analyst's expectations.
In government bond trading, the yield on the 10-year note edged up to 2.59 percent from 2.58 percent Friday.
In commodities trading, the price of oil fell 79 cents, or 0.79 percent, to $100.30 a barrel. Gold rose 2.20, or 0.2 percent, to $1,317 an ounce.
Among other stocks making big moves, Tellabs rose 9 cents, or 3.8 percent, to $2.44 after the communications equipment maker agreed to be acquired for $891 million by investment firm Marlin Equity Partners.