Short answers, pulled from the story.
The Cincinnati Red Stockings became baseball's first all-professional squad on the 4th of April 1869. This team marked a clear break from amateur play that had dominated the sport for decades before this moment.
Commissioner Kenesaw Mountain Landis permanently banned eight players including Shoeless Joe Jackson and Eddie Cicotte from Major League Baseball. These players were involved in intentionally losing the 1919 World Series between the Chicago White Sox and Cincinnati Reds despite being acquitted in court.
Jackie Robinson made his debut at Ebbets Field on the 15th of April 1947 before a crowd of 26,623 spectators. He joined the Montreal Royals farm club for the 1946 season as part of The Noble Experiment before joining the major leagues.
Three teams relocated between 1953 and 1955: the Boston Braves became the Milwaukee Braves, the St. Louis Browns became the Baltimore Orioles, and the Philadelphia Athletics became the Kansas City Athletics. Walter O'Malley moved the Brooklyn Dodgers to Los Angeles in 1958 marking the first major league franchise on the West Coast.
The second strike occurred from the 12th of August 1994 to the 25th of April 1995 causing the cancellation of over 900 games and forfeiting the entire postseason. This labor dispute followed an earlier strike in 1981 that ran from June 12 until July 31 canceling 713 games.