Why are fireworks customary for Independence Day?

In a July 3, 1776 letter to his wife, John Adams declared that the signing of the Declaration of Independence should be a “great anniversary Festival” and “solemnized with Pomp and Parade, with Shews, Games, Sports, Guns, Bells, Bonfires and Illuminations from one End of this Continent to the other from this Time forward forever more.”

A year later, Congress itself ordained the tradition, enjoying in Philadelphia “a grand exhibition of fireworks, which began and concluded with thirteen rockets on the commons, representing the 13 states.  

U.S. News & World Report

Happy 4th of July !!