In 1939, in conjunction with a trend towards recognizing and honoring citizens, William Randolph Hearst set the stage for an official holiday through prominent discussions in his chain of daily newspapers. Following suit in 1940, Congress designated the third Sunday in May as I am an American Day.

In 1952, President Harry Truman officiated the holiday, signing a bill which actually changed both the name and date, but maintained the premise. With that, September 17th was declared Citizenship Day.  Some Americans continue to observe the May holiday, despite the change.

Tradition
Today, across the country, various civil and government groups gather together to discuss, honor and celebrate the privileges and responsibilities of US citizenship. The entire week, from September 13th through September 17th is US Constitution Week, so for the very patriotic--or the very zealous--this is a week-long affair.

The purpose of this holiday is to honor both, native-born and naturalized foreign-born citizens. In 1939, Randolph Hearst gave the day national prominence through his chain of daily newspapers when a movement to recognize new citizens begun. 

Facts
Citizenship Day focuses on the rights and responsibilities of U.S. citizens both native-born and naturalized. The choice of September 17 for this observance commemorates the events of September 17, 1787 when the United States Constitution was singed by delegates from 12 states at the Constitutional Convention in Philadelphia, Pennsylvania. This day celebrates our Supreme Law of the Land as the oldest working Constitution in the world.