The best answer is “because it's always been celebrated on the 4^th of July.” That's not the right answer, just the best answer.

Richard Henry Lee from Virginia proposed his resolution declaring the colonies to be independent from the British Empire on June 7^th, 1776. He was actually following directions from President Edmund Pendleton of the Virginia Convention and Lee used almost verbatim Edmund's instructions in crafting his resolution.

It took several weeks to round up enough support before the resolution was passed on July 2^nd. Announcements of the passage appeared in the Pennsylvania Evening Post (the first newspaper to be printed in the colonies, not to be confused with Benjamin Franklin's Saturday Evening Post) and the Pennsylvania Gazette. The headline in the Pennsylvania Evening Post read: “This day the Continental Congress declared the United Colonies Free and Independent States.”

Once Lee's resolution was passed by the Second Continental Congress its attention was focused on the official statement, the Declaration of Independence, to explain to all the world the reasons for the separation. To that end a Committee of Five were nominated to draft it, including Thomas Jefferson who did most of the writing.

Interestingly, Jefferson borrowed much of the language of his Declaration from the Mecklenburg Declaration proclaimed by the Scotch-Irish Presbyterians of North Carolina on May 20^th, 1775, nearly a year earlier. Here's some of the language from that declaration:

We do hereby dissolve the political bands which have connected us with the mother country, and hereby absolve ourselves from all allegiance to the British crown …

We hereby declare ourselves a free and independent people; are, and of right ought to be, a sovereign and self-governing association, under control of no power other than that of our God and the general government of Congress; to the maintenance of which we solemnly pledge to each other our mutual cooperation and our lives, our fortunes and our most sacred honor …

Once the committee's draft was presented it was subjected to all manner of debate and compromise and changes in the wording. On that day, July 4^th, Thomas Jefferson, John Adams and Benjamin Franklin signed it. However most of the rest of the signers didn't affix their signatures to the document until August 2^nd, 1776. Some didn't get around to signing it until January of 1777. However, the date of the document itself, July 4^th, is most likely the reason that that date is the one we use today to celebrate our independence.

On July 4^th, 1777, Philadelphia started the tradition with its celebration that included an official dinner for the Continental Congress, 13-gun salutes, speeches, prayers, music, parades, troop reviews, and fireworks. Ships in the harbor were decked out with red, white, and blue bunting. No state officially recognized July 4^th as a state celebration until 1981, when the Massachusetts General Court made it a state holiday.

There are a number of myths surrounding Independence Day, including the ringing of the liberty Bell, which cracked as it was first being rung. Another is Betsy Ross sewing the first flag of the United States. But there is one fact that historians often overlook: On July 4^th, 1826, exactly fifty years after the adoption of the Declaration, John Adams and Thomas Jefferson both died.

Enjoy your Fourth!

—————————

Sources:

Top 5 Myths About the Fourth of July!

History surrounding Independence Day in the United States

Bio of Richard Henry Lee

The Lee Resolution

Bio of Edmund Pendleton

The Reformed Doctrine of Predestination by Loraine Boettner (see esp. pages 387-388)