All facts are from http://www.canadianbaseballnews.com/:
May 23, 1901: Nap Lajoie becomes the first batter to be intentionally walked with the bases loaded.
May 24, 1935: The first-ever night game is played at Crosley Field in Cincinnati. President Franklin Delano Roosevelt throws the switch at the White House to turn on the lights.
May 24, 1964: Harmon Killebrew hits the longest measured home run at Memorial Stadium in Baltimore.
May 25, 1951: Willie Mays makes his MLB with the New York Giants.
May 25, 1966: Curt Flood hits the first grand slam at Busch Memorial Stadium in St. Louis.
May 26, 1977: Rickey Henderson of the Modesto A’s ties the minor league record, stealing seven bases.
May 27, 1968: MLB owners vote to award a National League franchise to San Diego.
May 28, 1941: The first night game is played at Griffith Stadium in Washington, DC, with the Yankees defeating the Senators; 5 years later, on May 28, 1946, the first night game is played at Yankee Stadium, with the Senators beating the Yankees.
May 28, 1957: MLB owners grant permission to the Brooklyn Dodgers and the New York Giants to relocate to the West Coast.