Fed Chair Janet Yellen speaks just before the closing bell Friday, and you can bet Wall Street will delay the start of its weekend to listen.

While Yellen's comments are always important, the markets have been fine-tuning positions for more than a week now after the Fed's shift in policy. With the laundry list of catalysts that traders have attributed to volatile moves in bonds, stocks and currencies this week, one undercurrent they all come back to is the central bank.