MLB players wear the number 21 to honor Roberto Clemente, a legendary baseball player and humanitarian. September 15th is Roberto Clemente Day in Major League Baseball, commemorating 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 symbol of respect for Clemente's number throughout his career. Additionally, players of Puerto Rican heritage and nominees or winners of the Roberto Clemente Award have the option to wear the number 21 on their jerseys. This tradition celebrates Clemente's legacy as a Hall of Famer, a Puerto Rican sports trailblazer, and a humanitarian who died tragically in a plane crash delivering aid to earthquake victims in 1972.