WASHINGTON—President Donald Trump is set to celebrate the nation’s 250th anniversary on the National Mall in front of the Washington Monument with a speech scheduled at 9:45 p.m. ET, followed by what organizers are describing as the world’s largest fireworks show. The night sky will glow for about 40 minutes, with approximately 850,000 pyrotechnic shells launched from the reflecting pool near the Lincoln Memorial, West Potomac Park, and barges floating on the Potomac River, setting a world record for the most fireworks launched in a single display. The show is estimated to cost about six times the amount normally budgeted for Fourth of July fireworks in the district, based on wholesale prices of approximately $1.5 million or more for such elaborate displays....