Jays (and other MLB teams) are wearing the number 21 to honor Roberto Clemente on Roberto Clemente Day, which is observed around September 15 each year. This day celebrates the life and achievements of Roberto Clemente, a Puerto Rican Hall of Famer known for his outstanding baseball career and humanitarian work. MLB players, managers, coaches, and umpires wear a "21" patch on their uniforms to commemorate Clemente, who wore No. 21 throughout his career. Additionally, players of Puerto Rican heritage, past Roberto Clemente Award recipients, and nominees can wear No. 21 during games on this day. The Toronto Blue Jays, including players like Vladimir Guerrero Jr., participate in this to honor Clemente's legacy.