Constitution Day and Citizenship Day is observed each year on September 17 to commemorate the signing of the Constitution on September 17, 1787, and “recognize all who, by coming of age or by naturalization, have become citizens.”
This commemoration had its origin in 1940, when Congress passed a joint resolution authorizing and requesting the President to issue annually a proclamation setting aside the third Sunday in May for the public recognition of all who had attained the status of American citizenship. The designation for this day was “I Am An American Day.”
In 1952 Congress repealed this joint resolution and passed a new law moving the date to September 17 to commemorate “the formation and signing, on September 17, 1787, of the Constitution of the United States” but the day was designated “Citizenship Day” and also retained its original purpose of recognizing all those who had attained the status of American citizenship. This law urged civil and educational authorities of states, counties, cities and towns to make plans for the proper observance of the day and “for the complete instruction of citizens in their responsibilities and opportunities as citizens of the United States and of the State and locality in which they reside.”
In 2004 under Senator Byrd's urging, Congress changed the designation of this day to "Constitution Day and Citizenship Day" and added two new requirements in the commemoration of this Day. The first is that the head of every federal agency provide each employee with educational and training materials concerning the Constitution on September 17th. The second is that each education institution that receives federal funds hold an educational program on the Constitution for students on September 17 of each year.
Source: Library of Congress