It is the anniversary of the day on which the Declaration of Independence was adopted by the Continental Congress - July 4, 1776...announcing to the world that the 13 colonies no longer belonged to Great Britain. Independence Day was first observed in Philadelphia on July 8, 1776.

On July 4, 1777, the night sky of Philadelphia lit up with the blaze of bonfires. Candles illuminated the windows of houses and public buildings. Church bells and volleys from ship cannons broke the quiet. The city was celebrating the first anniversary of the founding of the United States.

In 1941, Congress declared July 4 a federal legal holiday.

Who put the crack in the Liberty Bell?
The Liberty bell arrived in Philadelphia on September 1, 1752, but was not hung until March 10, 1753, on which day Isaac Norris wrote, "I had the mortification to hear that it was cracked by a stroke of the clapper without any other violence as it was hung up to try the sound." The cause of the break is thought to have been attributable either to flaws in its casting or, as they thought at the time, to its being too brittle.
There is disagreement about when the first crack appeared on the Bell. However, it is agreed that the final expansion of the crack which rendered the Bell unringable was on Washington's Birthday in 1846.
Now, on every Fourth of July, the bell is rung (symbolically tapped), in unison with thousands of bells across the nation.