Jackie Robinson Day is an annual “event…in Major League Baseball, commemorating and honoring the day Jackie Robinson made his major league debut, in 1947. Initiated for the first time on April 15, 2004, Jackie Robinson Day is celebrated each year on that day.” The uniform number 42 has been retired by every team since 1997, though any player wearing it at the time was able to keep it until he retired, e.g. Mariano Rivera of the New York Yankees. From 2007 on , though, one or more players on each team (and sometimes the WHOLE team) wear number 42 on April 15. Check out I Am 42.
As I noted way back here and contrary to reports like this one, Robinson was NOT the “first African-American player in Major League Baseball.” But since there was such a gap, because of racist players such as Cap Anson, who refused to play with blacks, Robinson’s accomplishments are in no way diminished.
My father was a lifelong Dodgers fan because of Jackie, while I rooted for the Yankees, one of the last two teams, along with the Boston Red Sox, to have a black player in the modern era. My rooting interest came from geography, my father’s from history.