Many of the colonists did not want to be ruled by Great Britain any more.
On the 4th of July in 1776, the Declaration of Independence was approved. By approving that document, our country declared (announced) that we would no longer be ruled by Great Britain. The 4th of July is known as the birthday of the United States of America. Here's more information about the signing of the Declaration of Independence.
Here's a historical fiction video about the first Fourth of July.