Tug McGraw loved baseball. He also loved St. Patrick's Day. While the year is disputed by some, it was sometime around 1981 when McGraw decided to combine the two loves and it led to a tradition. McGraw had always quipped about how he would tell the manager that he would do whatever he wanted him to do on March 17th, but wouldn't be available for anything on the 18th, because he simply would be in no condition to do anything.
McGraw stepped onto the field on St. Patrick's Day of 1981 - again, some dispute the year - wearing an all green uniform that he had dyed that color the night before. Umpire Nick Colosi, citing a major league rule that players on a team must all wear the same color uniforms, wouldn't allow McGraw to pitch. McGraw would have to have his own celebration away from the field, but in 1989, the Phillies decided to go all out and the entire team came onto the field in green uniforms, including green striped pants. It quickly became a tradition, although the green striped pants have been dropped, because they're apparently tougher to come by than the jerseys and hats. Other teams quickly followed suit and have worn some variation, either green caps, green jerseys or both.
The Phillies took it a step further. They have gone so far as to request that they always play on their home field on March 17th. They have also auctioned off the jerseys with the proceeds going to charity. Many of the fans in attendance also have taken to wearing green to the ballpark on St. Patrick's Day.
The tradition has taken on even more significance with the passing of McGraw in early 2004. Ironically, it was just before St. Patrick's Day of 2003 when McGraw fell ill and was diagnosed with a malignant brain tumor. The prognosis was extremely bleak, with doctors not expecting McGraw to live much past Opening Day of the 2003 season. The ever optimistic McGraw charged into a treatment program and would live another ten months, often visiting the team whenever possible. McGraw passed away on January 4, 2004, but the tradition that he gave birth to in the early '80s lives on and has become a tradition for many other teams around Major League Baseball.