On November 22, 2011, a new contract between Major League Baseball and its players union was ratified, and as a result, an expanded playoff format adding two clubs will be adopted no later than 2013 according to the new Collective Bargaining Agreement. The new format was finalized for the 2012 season on March 2, 2012, and will use the 2–3 game schedule format for the Division Series for the 2012 season only. The restriction against divisional rivals playing against each other in the Division Series round that had existed in previous years was eliminated, as the Baltimore Orioles and New York Yankees squared off in one of the best-of-5 LDS series in the American League. The stateside portion of the regular season started April 4 in Miami with the opening of the new Marlins Park, as the newly renamed Miami Marlins hosted the defending World Series champion St. Louis Cardinals. The regular season ended on Wednesday, October 3. The entire master schedule was released on September 14, 2011.
The Major League Baseball postseason was expanded to include a second wild card team in each league beginning in the 2012 season. The season marked the last for the Houston Astros as a member of the National League. Following the sale to new owner Jim Crane, the Astros agreed to move to the American League effective in the 2013 season, and would be assigned to the American League West, joining their in-state rivals, the Texas Rangers.
The Major League Baseball All-Star Game's 83rd edition was held on July 10 at Kauffman Stadium in Kansas City, Missouri, with the National League winning the All-Star Game for the third consecutive year in an 8–0 shutout of the American League. With the win, the National League champion earned home field advantage for the World Series, which began on October 24 and ended on October 28 when the San Francisco Giants swept the Detroit Tigers. The Civil Rights Game was held on August 18 at Turner Field, as the Los Angeles Dodgers beat the host Atlanta Braves, 6–2.