On Wednesday, October 4 at 2:20 p.m. ET, every TV, radio and cellphone in the United States should blare out the distinctive, jarring electronic warning tone of an emergency alert, accompanied by a notice along these lines: 

“This is a nationwide test of the Emergency Alert System, issued by the Federal Emergency Management Agency, covering the United States from 14:20 to 14:50 hours ET. This is only a test. No action is required by the public.”