The Fed prefers to announce any moves before they make them to avoid shocking the financial markets. That gives them time to plan for them. Sometimes saying you might do something then can have the same impact as the action itself and less of an actual responce is needed to achieve the same desired result. But as we have also seen in the last few years, sometimes the announcement and even the act itself has zero impact on the economy- in this I am refering to their "stimulus" which tried to get the economy going again.