The FOMC, which has held the benchmark interest rate at near zero since late 2008, is expected to continue that policy for an extended period while tapering its monthly asset purchases another $10 billion, to $55 billion.
But Yellen could signal a change in the threshold that the Fed has said would mark when it would start considering hiking the main interest rate from near zero.
Yellen could indicate "a broader range of economic indicators is being discussed, such as underemployment, the labor participation rate and inflation, rather than focusing strictly on unemployment, currently at 6.7 percent, and the target at 6.5 percent. We're very close to that," said Mangus.
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"I think the market could look at that in a positive way if it (the Fed) was looking at a broader range, since there are a lot of areas of the economy that are still recovering," Mangus added.
"Not that she is going to light it up with her comments, but it may signal a different way of delivering the Fed's message, so for that we are very interested," Kevin Giddis, head of fixed income capital markets at Raymond James, said in emailed comments.
The central bank's decision-making seems "pretty straightforward, no moves on rates and the continuation of the reduction of stimulus," noted Giddis.
"We don't want to hear that they are not doing it, primarily because we need to get out of this painted corner," Chip Cobb, portfolio manager at BMT Asset Management, said of the Fed's tapering program.
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"We want to hear at least that the economy is headed in the right direction; we don't want to hear the Fed is more concerned than the last meeting," Cobb said.
Yellen "seems to be the fairy godmother of the bull market because stock prices tend to rise on days that she speaks publicly about the economy and monetary policy," noted Ed Yardeni, chief investment strategist at Yardeni Research.
"I doubt she will say anything unexpected to rattle the market but after two consecutive days of big gains, 'a sell on the news' might be a scenario that could play out," offered Elliot Spar, market strategist at Stifel, Nicolaus & Co.
"By the way, I hate predicting the market on a Fed day," Spar added.
—By CNBC's Kate Gibson