I thought I'd post this just for fun. Even though I live just outside of NYC, I hadn't heard that there was a name for this not-so-unusual phenomenon or that people were treating this like an actual occasion. But apparently, just about anything is a good excuse to gather and gawk: http://doubtfulnews.com/2013/05/manhattanhenge-an-annual-tradition/
The kid on the video explained that living in central Manhattan, it's so rare that you actually can see a sunset. It's not quite as much of an occasion as a primitive man seeing his first eclipse. But it seems to me like they are treating it as an equivalent event.