The Second Continental Congress adopted the Declaration of Independence on July 4, 1776, whose 250th anniversary is America250.
Congress approved the final wording on July 4 after debating and revising a draft prepared primarily by Thomas Jefferson. The document announced that the thirteen American colonies regarded themselves as independent states rather than colonies subject to British rule.
The date is printed on the Declaration and became the basis for Independence Day. However, the independence decision itself came earlier: Congress approved the Lee Resolution on July 2. The parchment was not signed by all delegates on July 4; the formal signing ceremony is generally associated with August 2, 1776.
These different dates explain why historical accounts distinguish the vote for independence, the adoption of the Declaration, and the signing of the engrossed parchment. America250 uses July 4 because it is the document's date and the established national anniversary.