Starting in 2004, MLB officially began recognizing April 15 as Jackie Robinson Day, taking time to honor Robinson’s life and legacy on the field before every game.
In 2007, Ken Griffey, Jr.—one of the biggest stars in baseball at the time—asked the Commissioner’s office for special permission to wear 42 on Jackie Robinson Day, to honor Robinson further.
Commissioner Bud Selig loved the idea so much, that other teams were encouraged to allow players to don the number as well. In 2009, every player, coach, and even the umpires, wore 42 to honor Jackie, which is still the case today.