In the relatively recent past, many of us have forgotten that this country, unlike most, had a definite starting date, and that date is celebrated today: July 4, 1776 (the Declaration of Independence was basically agreed to on the second, but final revisions were in order and the document as we know it was announced on the 4th).
It was an act of imagination and courage. You have to have some nerve to declare your independence without an army, navy, or government.
Aside from the fireworks, it was a revolutionary moment not just in American history but world history as well.
Needless to say, bold as it was, it wasn't as if America was perfect. Slavery and the way the relationship with Native Americans evolved saw to that (though the relations with aboriginal peoples is not nearly as simple as the 20th/21st century point of view would have us believe. The history of New Amsterdam has some fascinating tales of interactions between the two worlds, and Washington's vision of how to live harmoniously with Native Americans, had it been accepted, would have changed things enormously).
One of the main points of the Declaration, and later expanded on in the Constitution, is that while man seeks freedom and liberty, human nature means that other people will try to take it away.
We weren't perfect then and we certainly aren't now. Still, we'll bet it's been a while since you actually read the damn thing. So we dare you - read it. Right now.
For these men to declare their colonies free from the British Empire was an act of immense courage and imagination, and it changed the world. The fireworks every year are great, but the real fireworks happened in Philadelphia, and they were remarkable. Still are.