Happy 4th of July (weekend).  This holiday has always signaled the start of summer for me.  I used to work at Disneyland and the 4th of July brought visitors from all over the world, the weather was almost always hot in Anaheim (where Disneyland is located) on this date, and it truly felt like the start of summer.

According to Wikipedia:  In the United States Independence Day, commonly known as the Fourth of July, is a federal holiday 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, political speeches and ceremonies and various other public and private events celebrating the history, government, and traditions of the United States.  ~via Wikipedia

Pictures via Veranda Magazine & Martha Stewart