It’s Fourth of July weekend in the United States — and this one is special.Saturday marks the nation’s 250th birthday, and Americans are marking the milestone with cookouts, parades, fireworks and other traditions.In the nation's capital, celebrations are set to culminate with a speech from President Donald Trump and a record-breaking fireworks show on the National Mall.Complicating matters, however, is a dangerous heat wave that has been shattering records, straining the electric grid and raising the risk for millions of people who choose to celebrate outdoors.