Among key S&P 500 sectors, energy, consumer discretionary and telecom rose, while consumer staples fell.

The dollar [ .DXY 81.44  +0.22 (+0.28%) ] gained slightly against a basket of foreign currencies as the euro struggled against the Swiss Franc amid continuing euro zone debt concerns. Gold remained a strong safe-haven alternative Monday, gaining nearly $7 to close at $1,385.50 an ounce, amid tensions in Korea and concerns about Europe.

Monday's market weakness likely reflected a bit of nervousness heading into the new year, as investors waited for important economic news coming out later this week, including Gross Domestic Product and existing homes sales on Wednesday, Doug Roberts, chief investment strategist at ChannelCapitalResearch.com told CNBC.com.

But Roberts expects stocks will continue to rise as investors seek to put money that had been on the sidelines or in bonds to work, and as the Federal Reserve continues to fuel the economy through bond purchases. The caveat, he said, is an unexpected jolt from outside the U.S., say, from a European bank, or actions by China, could rattle domestic markets.