The Dodgers wearing the number 21 jersey is related to MLB celebrating Roberto Clemente Day, which occurs every September 15. On this day, the league honors Roberto Clemente, a Hall of Famer and humanitarian who wore number 21. MLB allows nominees and previous recipients of the Roberto Clemente Award to wear the number 21 jersey. Additionally, players of Puerto Rican heritage can also wear the number 21 to honor Clemente. For the Dodgers, players like Mookie Betts (a nominee for the Roberto Clemente Award) have worn number 21 as part of this tribute. The team also includes other players who normally wear number 21, such as Walker Buehler and Kiké Hernández, who also wore it for this celebration. So, the Dodgers are wearing 21 to honor Roberto Clemente and highlight those nominated for the Roberto Clemente Award, which recognizes outstanding character, community involvement, and philanthropy in baseball.