July 4 was the day chosen as the founding of the United States of America in 1776 and the issue of the Declaration of Independence.
Doc Brown entered this date along with the year 1776 to demonstrate that one could witness the signing of the Declaration, granted that they were in Philadelphia.
Behind the scenes
However if one was able to reach Philadelphia on this date, they would have only seen John Hancock signing it. The other 55 delegates did not sign the document until August 2, even though the document was dated July 4, 1776.
- Back to the Future (Mentioned only)