Most cities with a grid street system have streets aligned with true north, south, east and west. As a result, most commuters know when the equinox nears because the sun sets directly in one's eyes.
Since Manhattan's grid system is aligned in the direction the island points, its east-west streets diverge about 30 degrees from true east and west. This results in Manhattanhenge, which occurs when the sun sets in line with the canyons of buildings on east-west cross streets. Manhattanhenge happens twice a year on the days around May 28 and (roughly) July 12.