Well, first of all: Happy Birthday, USA! And a happy 4th of July, y' all! I envy you to have your "National Day" in July. The NYT has the answer why (my emphasis; Wikipedia link added):
The signers of the Declaration of Independence had a lot on their minds, the least of which was our need, 230 years later, for a good midsummer holiday. The rhythms of our lives are awfully distant from the rhythms of theirs. But what you celebrate comes to depend, in time, on when you celebrate it. The Declaration was fundamentally a political event, a short, sharp document that made a powerful statement about human nature and human expectations. But the fact that it was dated July 4 guaranteed that in time, given the practical nature of Americans, it would be celebrated with hot dogs down by the lake while the sky erupted overhead.
And the NYT editorial also sums up what (in my opinion) is so special about this day (my emphasis):
The Fourth of July might seem like (...) a day to worry about the very real and practical matter of keeping our democracy alert, alive and undiminished. But this is also the best of days to admire the long continuity of our history, the profound American talent for compromise, the simple beauty of what endures. It's a good day to hope that every part of this country gets what it needs — a dry night in the Northeast, a long morning rain in the Rocky Mountain West, and truly thunderous fireworks in a clear sky just after dark.
I want to end this post with a pointer to a pointer: Daniel Drezner refuses to answer the question "[W]as American independence a good idea?", which is being discussed by Matthew Yglesias and Tyler Cowen, and he leaves the answer to Jefferson. Very clever, Dan ;-).