So far, U.S. markets are playing it safe, with all stock indexes trading close to flat. The Dow, S&P 500 and Nasdaq have all been bouncing between very small gains and losses on Thursday.

The real action should come at 2 pm ET when the Federal Reserve tells the world whether it will raise interest rates for the first time in nearly a decade. And then Fed chief Janet Yellen will explain the decision and offer hints on future policy during a televised press conference at 2:30 pm.

The outcome of these crucial events could have a big impact on stocks, bonds, currency and overseas markets not just on Thursday, but for months to come.

It's "the most widely debated, dissected, analyzed and now traded capital markets event in my 25 years on the Street," Nicholas Colas, chief market strategist at ConvergEx, wrote in a note to clients.