The streets of New York are arranged in a mostly-square grid. Once a year, the grid coincides with the path of the sun. This makes the sun directly visible down an entire street, creating a memorable image.
Alluding to Stonehenge, residents have started calling this day Manhattan-henge. Whatever you call it, it's striking to see. I wasn't able to be in Manhattan yesterday but here's a photo someone else took. What do you think?