A dangerous heatwave upended Fourth of July celebrations across swathes of the central and eastern United States on Friday, forcing officials in the nation’s capital and elsewhere to cancel or postpone dozens of parades, concerts and fireworks displays. Among the events disrupted by the sweltering heat was the Great American State Fair on the National Mall in Washington, a centrepiece of President Donald Trump’s efforts to mark the nation’s 250th birthday. The fair, designed to showcase all 50...