U.S. stock markets experienced a notable rally, marking the best performance on a Federal Reserve announcement day since July. The Dow Jones Industrial Average surged by 400 points, while the S&P 500 also achieved its best Fed day performance in the same timeframe. This rally was attributed to Federal Reserve Chair Jerome Powell's 'nonchalant' demeanor during his remarks, which appeared to reassure investors. As a result, yields on government bonds declined significantly, contributing to the positive sentiment in the markets.