Congress passed the Lee Resolution two days before adopting the Declaration of Independence.
Richard Henry Lee of Virginia introduced the resolution on June 7, 1776. Its central proposal stated that the United Colonies should be free and independent states, that their political connection with Great Britain should be dissolved, and that a plan of confederation should be prepared.
Congress approved the independence resolution on July 2, 1776. Two days later, on July 4, it adopted the Declaration, which explained the reasons for the break and presented the political case to a wider audience. This is why July 2 and July 4 have different historical roles in the independence process.
The Lee Resolution was not the Declaration itself. It was the formal decision to pursue independence, while the Declaration supplied the public justification. John Adams believed July 2 would become the great annual celebration, but July 4 ultimately became the commemorated date.