Article about Ken Singleton being called up to the majors on June 23, 1970
"....[The Mets] brought up Ken Singleton, crashing switch-hitter who has been murdering minor league pitching. "HE'LL PLAY as soon as he gets here," said manager Gil Hodges. "You don't bring up kid like that unless you intend to play him.""
He played his first game, the next day, June 24, 1970 going 0-4 in the second game of a double header against the Cubs.
From the "Daily News", New York, New York - Wednesday June 24, 1970. Pages 128 and 133 (via Newspapers.com)