"The second day of July, 1776, will be the most memorable epoch in the history of America. I am apt to believe that it will be celebrated by succeeding generations as the great anniversary festival. It ought to be commemorated as the day of deliverance, by solemn acts of devotion to God Almighty. It ought to be solemnized with pomp and parade, with shows, games, sports, guns, bells, bonfires, and illuminations, from one end of this continent to the other, from this time forward forever more."

Technically the legal separation of the Thirteen Colonies from Great Britain in 1776 actually occurred on July 2, 1776 when the Second Continental Congress voted to approve a resolution of independence that had been proposed the previous month by Richard Henry Lee of Virginia. The resolution called for a Declaration that the United States were now independent from Great Britain's rule. After voting for independence, Congress turned its attention to the Declaration of Independence, a statement explaining this decision. A Committee of Five of those in attendance at the meeting were tasked with preparing this declaration, but Thomas Jefferson was selected as its principal author. Congress debated and revised the wording of the Declaration. So while the decision to become independent of Great Britain was passed on July 2nd, it took another two days before the Declaration of Independence was finally approved. That motion was passed on July 4.
Adams got everything else right, except the day on which the celebration would be marked. From the outset, Americans celebrated independence on July 4, because this was the date shown on the much-publicized Declaration of Independence, rather than on July 2, the date the resolution of independence was approved in a closed session of Congress.
There is still some debate among historians as to whether members of Congress signed the Declaration of Independence on July 4. There is considerable evidence that it was in fact signed on that day. Thomas Jefferson, John Adams, and Benjamin Franklin all later wrote that they had signed it on that day. Some other historians have concluded that the Declaration was signed nearly a month after its adoption, on August 2, 1776, and not on July 4 as is commonly believed. In 1796, signer Thomas McKean wrote that some signers were not present in Philadelphia on July 4th. Several were not even elected to Congress until after that date.
In 1821 the Secret Journals of Congress were published. They contained two previously unpublished entries about the Declaration. The Secret Journals entry for July 19 reads: "Resolved That the Declaration passed on the 4th be fairly engrossed on parchment with the title and stile of 'The unanimous declaration of the thirteen united states of America' & that the same when engrossed be signed by every member of Congress." The entry for August 2 states: "The declaration of Independence being engrossed & compared at the table was signed by the Members."
In 1884, historian Mellen Chamberlain wrote that these entries confirm that the famous signed version of the Declaration had been created following the July 19 resolution, and was not signed by Congress until August 2. Subsequent research suggests that many of the signers were not present in Congress on July 4, and that some delegates may have added their signatures even after August 2.
However both Jefferson and Adams always stated that the signing ceremony took place on July 4. But even renowned historian and Adams biographer David McCullough concludes that Adams is mistaken. He wrote" "No such scene, with all the delegates present, ever occurred at Philadelphia."
In 1986 legal historian Wilfred Ritz concluded that about 34 delegates signed the Declaration on July 4, and that the others signed on or after August 2. Ritz argues that it was implausible that Adams, Jefferson and Franklin are all mistaken. Ritz concludes that historians had misinterpreted the July 19 resolution. According to Ritz, this resolution did not call for a new document to be created, but rather for the existing one to be given a new title, which was necessary after New York had joined the other 12 states in declaring independence. Ritz reasoned that the phrase "signed by every member of Congress" in the July 19 resolution meant that delegates those who had not signed the Declaration on the 4th were now required to do so.
Whichever scenario is correct, Independence Day is a national holiday marked by patriotic displays. It is a federal holiday, and all non-essential federal institutions are closed. Independence Day is marked with fireworks displays and patriotic songs such as the national anthem, "The Star-Spangled Banner"; "God Bless America"; "America the Beautiful"; "My Country, 'Tis of Thee"; "This Land Is Your Land"; "Stars and Stripes Forever"; and, regionally, "Yankee Doodle" in northeastern states and "Dixie" in southern states.

Coincidentally, both John Adams and Thomas Jefferson, the only signers of the Declaration of Independence later to serve as Presidents of the United States, died on the same day: July 4, 1826, which was the 50th anniversary of the Declaration.