Original source at NASA (which, as usual, phys.org does not link to, they even ripped the short movie from youtube)
http://www.nasa.gov/feature/goddard/2016/nasa-camera-shows-m...
http://www.nasa.gov/feature/goddard/2016/nasa-camera-shows-m...
NASA's own press release at least specifies how unusual this event is, and the answer is "a bit less unusual than new years"