Independence Day, commonly known as the Fourth of July, is a federal holiday in the United States commemorating the adoption of the Declaration of Independence on July 4, 1776, declaring independence from the Kingdom of Great Britain. Independence Day is commonly associated with fireworks, parades, barbecues, carnivals, fairs, picnics, concerts, baseball games, family reunions, and political speeches and ceremonies, in addition to various other public and private events celebrating the history, government, and traditions of the United States. Independence Day is the national day of the United States. (source: Wikipedia)
This cartoon by Bob Englehart from The Hartford Courant shows the mascots of the two main US political parties: the Democratic donkey and the Republican elephant against the backdrop of the Declaration of Independence. They begrudgingly wish each other a "Happy July 4th" — a comment on the hostile relations between the two parties in this crucial election year.
Ya is used in writing as a way of showing the way people sometimes pronounce the word ‘you’ or ‘your’.