That is the day that the US gained Independence from Britain. That's the official day of signing in 1776.
July 4th is considered the birthday of the United States, the date of the adoption of the Declaration of Independence in 1776.
July 4, 1776, was the day America declared its independence from the British. On July 4th two founding fathers signed the Declaration of Independence, days and weeks later it was signed by the rest.
The Fourth of July is celebrated by Americans as Independence Day.