From the NYT:
“Bright Lights, Big City†takes on a whole new meaning during Manhattanhenge, the semiannual phenomenon when the sun so perfectly aligns with the Manhattan grid that it shines on both sides of every cross-street. (May 31, 8:17 p.m., and July 11, 8:25 p.m.; the best views are from the east and from the Empire State and Chrysler Buildings.)
As long as I’ve worked in the city, I never witnessed this myself. It’s probably more stunning if you are in a cab going North or South and seeing the sunset pop into view at every cross street.