Now that we’re in October, sunset has moved into the 6:30 range, but “civil” twilight is hovering around 7:00. So it is appropriate that from October 3-8, the High Line will be hosting The Mile-Long Opera, a biography of 7 o’clock, beginning each night at seven. The free presentation consists of one thousand singers from across New York, delivering the world premiere of this site-specific event, as the audience makes their way along the elevated park. The words were written by poets Anne Carson (librettist) and Claudia Rankine (essayist), based on interviews conducted with New Yorkers at Abrons Arts Center and the Greater Harlem Chamber of Commerce in Manhattan, ARTs East NY in Brooklyn, Jacob A. Riis Neighborhood Settlement in Queens, the POINTCDC in the Bronx, and Snug Harbor in Staten Island, discussing what seven o’clock means to them. The work was created by composer David Lang and Diller Scofidio + Renfro, the design studio behind the High Line. The Mile-Long Opera is directed by Elizabeth Diller and Lynsey Peisinger, with music direction by Donald Nally, sound by Jody Elff, lighting by John Torres, and costumes by Carlos Soto; wildly inventive, multidisciplinary Icelandic artist Ragnar Kjartansson is the creative adviser. Although advance registration is closed, there will be standby lines beginning at 6:30 each night at Gansevoort & Washington Sts.; since the event is free, you can expect many people who have signed up will not show, so there should be a pretty good chance of getting in. You can also experience the event in 360 degrees via an app that will be available on October 3. So think about it: Just what does 7:00 mean to you?