On this day in 1871, former Red Ned Cuthbert made his major league debut with the Philadelphia Athletics of the National Association. The Athletics went on to win the National Association with a 21-7 record. Those Athletics have no connection to the Athletics that we know today. The 1871 National Association was the world's first professional baseball league, giving way to the National League in 1876. May 20 must have been one of the first days of games as a number of 19th century players made their debut on this day.
Al Reach is another player who made his big league debut with the Athletics on this day. Reach never played for a Cincinnati team, but is an important figure in baseball history for his publication of Reach's Official Base Ball Guide (commonly known as the Reach Guide) from 1883 to 1939. Reach's main competitor in this endeavor was Albert Spalding's Spalding Guide. (Yes, that's the same Spalding who founded the sporting goods company.) The Reach Guide is considered to be the more thorough and detailed of the two guides (Baseball-Reference).
On this day in 1893, former Red Fritz Von Kolnitz was born in Charleston, SC (I wasn't expecting that one).
On this day in 1900, former Red George Grantham was born in Galena, KS.
On this day in 1909, the Reds sold right-hander Pat Ragan to the Cubs.
On this day in 1949, Don Newcombe made his major league debut with the Dodgers. The "Newk" is best known for his days as an ace in Brooklyn and winning the first Cy Young Award in 1955. He also won the MVP in 1955 and was Rookie of the Year in 1949. Newcombe pitched for Cincinnati from June 1958 to July 1960, in which time he compiled a 24 - 21 record with a 3.64 ERA (112 ERA+) in nearly 450 innings.
On this day in 1963, former Red David Wells was born in Torrance, CA.
On this day in 1966, Cincinnati sold lefty Gerry Arrigo to the Mets.
On this day in 1974, former Red and Cincinnati native Kent Tekulve made his big league debut with the Pirates.
On this day in 1976, former Red Ramon Hernandez was born in Caracas, Venezuela.
On this day in 1980, former Red Austin Kearns was born in Lexington, KY.
On this day in 1983, former Red Adam Rosales was born in Chicago.
On this day in 2005, Cincinnati signed righty Enerio Del Rosario.
On this day in 2013, the Reds signed Trevor Bell.