Screech, the Washington National's Mascot, poses with the starting lineup of Marines at Marine Corps day at Nationals Park on May 8, 2013. Marine Corps day is the first of five branch-themed days National's Park has scheduled for the season to recognize and show appreciation for the men and women who serve in the United States armed forces. - Screech, the Washington National's Mascot, poses with the starting lineup of Marines at Marine Corps day at Nationals Park on May 8, 2013. Marine Corps day is the first of five branch-themed days National's Park has scheduled for the season to recognize and show appreciation for the men and women who serve in the United States armed forces.