Every player in Major League Baseball wears number 42 today on Jackie Robinson Day, in honor of the anniversary of Jackie Robinson breaking the color line. The practice will be repeated in future years.

This Day In Baseball

On April 15, 2009 – – Every player in Major League Baseball wears number 42 today on Jackie Robinson Day, in honor of the anniversary of Jackie Robinson breaking the color line. The practice will be repeated in future years.

Source
Baseball Reference April 15