Think about your neighborhood. Think about how much of it taken up by parking spaces - not just marked parking spaces, but streets that are wider than they need to be for traffic so cars can park on one or both sides.
Now think about how many of those parking spaces are filled, even at peak usage times. If you're in Adams Morgan, parking's probably pretty efficiently used. But if you're in Germantown or Springfield or even many parts of Arlington, there are vast amounts of land taken up by little-used parking areas - land that could be used in myriad other ways.
Now think about what you could do with that unused land if it wasn't parking. Could you build a playground? Plant a garden? Put in some shade trees to make your summer walk to Metro a bit cooler? Open space for an off-the-charts adorable scene like this?
That's what Par(king) Day is all about. GreaterGreaterWashington.org has more pictures & video from celebrations around the region.