In Congress, July 4, 1776.
The Declaration of Independence is arguably one of the most influential documents in American History. Other countries and organizations have adopted its tone and manner in their own documents and declarations. For example, France wrote its 'Declaration of the Rights of Man' and the Women's Rights movement wrote its 'Declaration of Sentiments'. However, the Declaration of Independence was actually not technically necessary in proclaiming independence from Great Britain.
A resolution of independence passed the Philadelphia Convention on July 2. This was all that was needed to break away from Britain. The colonists had been fighting Great Britain for 14 months while proclaiming their allegiance to the crown. Now they were breaking away. Obviously, they wanted to make clear exactly why they decided to take this action. Hence, they presented the world with the 'Declaration of Independence' drafted by thirty-three year old Thomas Jefferson.
The text of the Declaration has been compared to a 'Lawyer's Brief'. It presents a long list of grievances against King George III including such items as taxation without representation, maintaining a standing army in peacetime, dissolving houses of representatives, and hiring "large armies of foreign mercenaries." The analogy is that Jefferson is an attorney presenting his case before the world court. Not everything that Jefferson wrote was exactly correct. However, it is important to remember that he was writing a persuasive essay, not a historical text. The formal break from Great Britain was complete with the adoption of this document on July 4, 1776.
Jefferson wrote the Declaration in a way that was meant to be read aloud. The first two paragraphs are some of the most powerful words ever written.