It's Independence Day here so I'm not writing much.
For the readers in other countries:
I know that in the United States we're pretty clueless about holidays in other countries. Many of us think that Cinco de Mayo (Fifth of May) in Mexico is like our Independence Day, but it's not. At least Canadian Thanksgiving is like our own Thanksgiving. And I really don't know how much people in other countries know about our holidays. So I'll give the short version.
Like many of you, we used to be part of Great Britain. A lot of people say it's the taxation that drove us to finally break away. It's more accurate to say that it was the fact that we had no say in how our taxes were collected or spent. There's a lot of other things we could point our fingers at and say "that's why they did it", but they all come down to taxation without representation.
On 2 July 1776, the Continental Congress voted to break away from Great Britain. They needed to explain to King George and the people in the colonies why they did so. They wrote the Declaration of Independence to do this. On 4 July they approved the wording of the Declaration. On 2 August it was officially signed, although some people signed later.
Clearly, there's a number of days on which we could celebrate our independence. I think we use the 4th of July because that's a date that many of those who drafted the Declaration of Independence accepted as the date we broke away.
The fireworks are a bit harder to explain. There was another war with England later on. They'd been kidnapping Americans off of ships and making them serve in the British navy. We fought back in what became the War of 1812. At a battle in Baltimore (where my girlfriend lives) Ft McHenry was being defended from invasion. Besides all the cannon balls that were fired back and forth there were rockets that look a lot like giant bottle rockets. They put on a great light show that is talked about in our national anthem, The Star Spangled Banner. So they're from a different war entirely, but still part of the patriotic mood of the day.