The number 21 on MLB uniforms is worn to honor Roberto Clemente, a legendary Puerto Rican Hall of Famer and humanitarian who wore No. 21 throughout his career. MLB has designated September 15 as Roberto Clemente Day, when players, coaches, and personnel, especially those of Puerto Rican descent and Roberto Clemente Award nominees or recipients, wear No. 21 or a patch with the number to commemorate his legacy. The Pittsburgh Pirates, Clemente's longtime team, also honor him by having players wear No. 21 on this day. This number symbolizes Clemente's outstanding achievements in baseball and his humanitarian contributions off the field.