Opening Day for most clubs was a day later on April 1. The regular season ended on September 30, extended one day for a one-game playoff between the Tampa Bay Rays and the Texas Rangers to decide the second American League Wild Card winner.
The 2013 season was the first with the Houston Astros as a member of the American League (AL), playing in the West Division. This marked the first growth in the number of American League teams since the 1977 Major League Baseball expansion added the Seattle Mariners and Toronto Blue Jays. When the then-named Tampa Bay Devil Rays were added to the AL in 1998, the Milwaukee Brewers were reassigned to the National League (NL) the same year, keeping the AL at 14 teams. With the Astros' move to the AL, giving both leagues 15 teams, interleague play occurred throughout the entire season for the first time in MLB history.
The Major League Baseball All-Star Game's 84th edition was held on July 16 at Citi Field, home of the New York Mets, with the AL defeating the NL, 3–0. With the win, home field advantage at the World Series was awarded to the AL.