Jackie Robinson Day is a event which occurs annually in Major League Baseball, commemorating and honoring the day Jackie Robinson made his major league debut. April 15 was Opening Day in 1947.

The first Jackie Robinson Day started in April 15, 2004. The festivity is a
result of Robinson's memorable career, best known for becoming the first black major league baseball player of the modern era in 1947.   His debut with the Brooklyn Dodgers (Los Angeles Dodgers) ended eighty years of baseball segregation, also known as the baseball color line, or color barrier.  Jackie also was inducted into the Baseball Hall
of Fame in 1962.

On that one day, all players, coaches, and managers
on both teams, and the umpires, wear Jackie's #42 on their jerseys. The #42 is only number that has been retired in the MLB.

All Clubs playing at home on Friday, April 15, 2016 will commemorate Jackie Robinson Day with special on-field, pre-game ceremonies in their ballparks.  Home Clubs will feature Jackie Robinson Day jeweled bases and lineup cards.

The Dodgers will host both Rachel and Sharon Robinson; the wife and daughter of Jackie Robinson, in a special pre-game ceremony that will include Jackie Robinson Foundation Scholars.
https://en.wikipedia.org/wiki/Jackie_Robinson_Day
http://m.mlb.com/news/article/172342410/mlb-announces-jackie-robinson-day-details