Just the data please! I was a bit shocked, as you may be as well, to see that all of the net gains in the S&P 500 index from June 30, 1997 through December 22, 2015 have come on Fed days. Since mid-1997, the Fed has met 149 times. The meetings typically last two days with the announcement made on the second day. The daily percentage change for the S&P 500 index on the day prior to the Fed announcement averaged 0.17% and the daily percentage change on the announcement date averaged 0.36%. On all of the other trading days, the S&P 500 daily average percentage change was 0.01%.
Here is what it looks like if you removed the Fed dates (right).