Why do some people believe May 21st is going to be Judgment Day?

I don’t know about how it is where you live, but this whole ‘the world is ending on May 21’ is kind of a thing here in the Bay Area. There are billboards all over and even Yelp’s newsletter has mentioned it.

So, tell me, a total heathen, what the significance of 5–21-11 is.

I thought we were doomed on 12–12-12, but apparently, we have only mere days to prepare!!!!!!!1