Congress formally approved the name “United States of America” on September 9, 1776, after it first appeared in the Declaration of Independence.
The Declaration’s original title referred to “the thirteen United States of America.” That wording helped establish the name in public and diplomatic use, but the Continental Congress did not formally approve the name until September 9.
This distinction is easy to miss because the Declaration is associated with July 4. July 4 was the date Congress adopted the Declaration’s final text, while September 9 was the later date on which Congress directed that the name “United States” replace “United Colonies” in official documents.
The naming decision belongs to the broader chain of events remembered by America250: independence was proposed, declared, explained, printed, signed, and then expressed through a formal national name.