August 15 is a day commemorating the founding of the United States of America. The date is also celebrated as Constitution Day. On this day, in 1776, the United States Constitution was signed by delegates from 13 states in Philadelphia. The United States Constitution is the cornerstone of the government of the United States. It defines the national government, its powers, and how it is organized. The Constitution also establishes a system of checks and balances, which guard against the accumulation of power in any one branch of government.