The beginning of interleague games in the MLB
Since its very beginning, interleague games in the MLB were only held during the following events: spring training, All Star Game, and the World Series. Except for those occasions, watching an interleague game was only a dream.
But such situation changed in 1997 (not long after the legendary strike by the players in 1994, that led to the cancellation of the MLB's postseason and also the World series of that year), when the MLB agreed to introduce games between teams belonging to different leagues during the regular season.
It was on June 12, 1997, when for the first time in 126 years of profesional baseball history, a game between teams coming from different leagues was played during the regular season: the Texas Rangers hosted the San Francisco Giants.
This decision led to new conditions that allowed fans to watch games between teams that caused much expectation because of playing in the same city or neigbouring places (e.g., NY Yankees vs. NY Mets, Chicago Cubs vs. Chicago White Sox, Cleveland Indians vs. Cincinnati Reds, Oakland Athletics vs. San Francisco Giants, etc.).
But games between teams belonging to different leagues or conferences were not new in sports, as the NFL has always promoted that kind of matches. But to allow this games increases the attendance and expectation for some MLB series that otherwise could not been seen for years, so I guess that this decision marked a new era in professional baseball history. Congratulations to all those leaders that had the vision to promote interleague games in baseball.