Image from The National Archives
Constitution Day commemorates the formation and signing of the U.S. Constitution by thirty-nine brave men on September 17, 1787, and recognizes all who are born in the U.S. or who by naturalization have become citizens.
From the Library of Congress Constitution Day website:
The members of the Constitutional Convention signed the United States Constitution on September 17, 1787 in Philadelphia, Pennsylvania. The Constitutional Convention convened in response to dissatisfaction with the Articles of Confederation and the need for a strong centralized government. After four months of secret debate and many compromises, the proposed Constitution was submitted to the states for approval. Although the vote was close in some states, the Constitution was eventually ratified and the new Federal government came into existence in 1789. The Constitution established the U.S. government as it exists today.