Here comes Janet Yellen ...

Friday is finally here, and so is Federal Reserve Chair Janet Yellen's biggest speech of the year.

At 10 a.m. ET, Yellen will deliver her address at the annual Jackson Hole symposium, titled "The Federal Reserve's monetary policy toolkit."

While the speech could dwell on the Fed's options to keep the economy going, Yellen could also use this as a chance to prep markets for imminent interest rate hikes.

In December, the Fed raised its benchmark interest rate for the first time since 2006. It has since signaled that another increase by 25 basis points would be appropriate by or at its December meeting, and traders are increasingly coming to believe that.

The Fed's next policy meeting is on September 14.

Yellen's speech "has the potential to move markets substantially, but that's not our core expectation," said Ian Shepherdson, chief economist at Pantheon Macroeconomics, in a preview. "It's more likely, we think, that Dr. Yellen will stick to the core FOMC view, which remains that "only gradual increases" in rates will be required, and that rates are "likely to remain, for some time, below levels that are expected to prevail in the longer run."

Stocks were higher ahead of Yellen's speech, with the S&P 500 up 8 points, or 0.39%, at 9:40 a.m. ET. Treasuries rallied as well, and the ten-year yield was down one basis point to 1.56%.

We'll have full details of Yellen's speech and the market reaction at the top of the hour.