On June 18, 2006, the Nationals played the Yankees in front of 45,157 fans -- the largest crowd to witness a baseball game at RFK Stadium. It was an afternoon game being played on Father's Day.
Zimmerman's dad, Keith, was in attendance, but didn't have much to cheer about for most of the game. The Nationals were held in check by Yankees pitcher Chien-Ming Wang, who allowed only one run in the first eight innings.
"I remember that Wang dominated us the whole game," said Zimmerman, who had scored the Nationals' only run. "So to be able to come up to the plate in the ninth with a chance to win was kind of surprising."
The Nationals had held the Yankees to two runs, so they trailed by only one going into the bottom of the ninth.
Yankees closer Mariano Rivera had pitched in two games in a row, and Wang was pitching well, so manager Joe Torre chose to leave him in the game. Nationals pinch-hitter Marlon Anderson reached base, setting up Zimmerman's at-bat.
Zimmerman had seen Wang's slider all day, so when one of them floated in the strike zone, it was his to hit. The home run landed in the Yankees' bullpen in left field, and Zimmerman trotted around the bases -- his first walk-off home run.
"I had never done anything like that in my career," he said. "Little league, college, anything. I don't think I'd even scored the winning run or anything like that."
The home run lifted the Nationals to a 2-1 series win over the Yankees, something fans remember to this day.
"I get asked about it a lot," Zimmerman said. "You don't really remember it. You just hit it, and then running around the bases you're not really thinking about anything, so it's kind of hard to tell people about it."
He added that having his family on hand to witness the moment made it even more special. Some lucky fan has a special souvenir of the event. After it landed in the Yankees' bullpen, one of their relief pitchers tossed the ball into the stands on the third-base line.
It also marked the first time since baseball returned to Washington that fans gave a player a curtain call, hanging around until Zimmerman re-appeared from the dugout at the urging of his teammates.
Two weeks later, the man who never hit a walk-off home run did it for the second time as the Nationals defeated the Florida Marlins.
His home run against the Yankees is the one that stands above the others, though. And while Zimmerman routinely finds himself on the highlight reel for his defense, he won't soon forget his game-winner.
"I take a lot of pride in defense," he said. "But to hit a home run to win the game, there's nothing like that. Then to run around the bases -- nothing else can beat that."