On this day the Continental Congress formally declares the name of the new nation to be the “United States” of America. This replaced the term “United Colonies,” which had been in general use.

In the Congressional declaration dated September 9, 1776, the delegates wrote, “That in all continental commissions, and other instruments, where, heretofore, the words ‘United Colonies’ have been used, the stile be altered for the future to the “United States.”

A resolution by Richard Henry Lee, which had been presented to Congress on June 7 and approved on July 2, 1776, issued the resolve, “That these United Colonies are, and of right ought to be, free and independent States….” As a result, John Adams thought July 2 would be celebrated as “the most memorable epoch in the history of America.” Instead, the day has been largely forgotten in favor of July 4, when Jefferson’s edited Declaration of Independence was adopted. That document also states, “That these United Colonies are, and of Right ought to be FREE AND INDEPENDENT STATES.” However, Lee began with the line, while Jefferson saved it for the middle of his closing paragraph.

By September, the Declaration of Independence had been drafted, signed, printed and sent to Great Britain. What Congress had declared to be true on paper in July was clearly the case in practice, as Patriot blood was spilled against the British on the battlefields of Boston, Montreal, Quebec and New York. Congress had created a country from a cluster of colonies and the nation’s new name reflected that reality.

http://www.history.com