September 17 is designated as "Constitution Day and Citizenship Day" to commemorate the signing of the U.S. Constitution in Philadelphia, which took place on September 17, 1787. It is intended to be a day "to reflect on the importance of active citizenship, recognize the enduring strength of our Constitution, and reaffirm our commitment to the rights and obligations of citizenship in this great Nation."
Source: U.S. Department of Education. Commemorating Constitutions Day and Citizenship Day. 19 Sept. 2022. https://www2.ed.gov/policy/fund/guid/constitutionday.html
"The final version of the Constitution, approved on September 17, 1787, established a federal democratic republic with an indivisible union of sovereign states. It is a democracy because people govern themselves, representative because people choose elected officials by free and secret ballot, and a republic because the government derives its power from the people. This model of government is based upon the idea of popular sovereignty, which upholds that the people are the only source of government's power. Furthermore, the government's power is limited because it can only do the things that people have authorized the government to do. The Constitution established a federal government with broad powers that were equally divided among the three branches, firmly establishing the principle of balanced government with a separation of powers and a system of checks and balances. Although powerful, the federal republic was created to uphold liberty, guaranteed because a division of power between the federal and state governments, known as federalism, would prevent one government entity from assuming too much power."