Try this neat little unit to see when something like this happens -
http://sourceforge.net/project/platformdownload.php?group_id=48857You can set the location you are at, and tell it any date or just click the fast forward a few times and you get an effect like The Time Machine and watch the changes in the sky, a few thousand years in a few seconds!
As for how often, pretty rare to have three local bodies in such close formation, lots of years at a guess to have the three brightest objects in the sky do it again.
