Once in a blue moon...

I am told that the bright shiny orb up there tonight (Aug 21) is a blue moon. There is this phrase, "Once in a blue moon..." One property ad, below, defines this as:

And when I got onto Google, it took me to searches for Claude Debussy's Clair de Lune. The image that came with this YouTube clip of Clair de Lune is appropriate:

http://www.youtube.com/watch?v=AzQTeCn5HBg

And here's the Marcels' original 1961 "Blue Moon":

http://www.youtube.com/watch?v=qu4OkmzqK2E

But, really, what is a blue moon? Here's Wikipedia's description of the phenomenon:

http://en.wikipedia.org/wiki/Blue_moon

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Finally, still on things blue, here are two creative blue-themed ads: