MLB players wear the number 21 to honor Roberto Clemente, a legendary Puerto Rican baseball player and humanitarian. September 15 is celebrated as Roberto Clemente Day across Major League Baseball, marking his remarkable career and selfless acts off the field. On this day, all MLB players, managers, and coaches wear a "21" patch on their uniforms as a tribute. Additionally, players of Puerto Rican heritage, past winners, and nominees of the Roberto Clemente Award have the option to wear the number 21 jersey. This tradition honors Clemente's impact on baseball and his humanitarian legacy, including his tragic death in a plane crash delivering aid to earthquake victims in 1972. The day also coincides with the start of National Hispanic Heritage Month, celebrating his contributions as a trailblazer and inspiration.
