Manhattanhenge is when the setting sun lines up with the east-west grid of Manhattan streets.
The event occurs on May 28th and July 12th or 13th
The street grid is rotated 29 degrees from geographic north.
If the grid was oriented to geographic north, the setting sun would line up at the spring and autumn equinoxes.
Day and night are the same length on the equinoxes, March 20 and September 22nd or 23rd.
Manhattanhenge was coined by Neil deGrasse Tyson, at the American Museum of Natural History.
NYCHenge has a map of when the setting sun lines up with other streets in New York City.