According to 36 U.S.C. 106, "September 17 is designated as Constitution Day and Citizenship Day" to commemorate the signing of the U.S. Constitution on September 17, 1787.
In 1952, a joint resolution (66 Stat. 9) was passed which designated September 17 as Citizenship Day. Then in 1956, another joint resolution (70 Stat. 932) established September 17 through 23 as Constitution Week. Public Law 105-225 revised and codified laws related to "Patriotic and National Observances" as Title 36 of the United States Code in 1998.
Finally, the Consolidated Appropriations Act of 2005 (Pub. L. 108-447 (very large file)) added "Constitution Day" to the law and mandated " the civil and educational authorities of States, counties, cities, and towns are urged to make plans for the proper observance of Constitution Day and Citizenship 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."