Wondering about the phrase "once in a blue moon" is like asking the old question, "Which came first? The chicken or the egg"?
Wikipedia, (which is kind of like talking to really entertaining people who "seem" to know a lot of stuff, but you are not completely sure of their accuracy) has tons of info on the history of blue moons. Check it out here.
One theory is that a blue moon is this: Most years have 12 full moons - one monthly. Each year contains roughly eleven days more than the lunar year , the extra days accumulate, so every two or three years there is an extra full moon.
As with most things, a lot of the blue moon story goes back to religion: "If they say the moon is belewe / We must believe that it is true" [If they say the moon is blue, we must believe that it is true].
All I can say is that it is beautiful here in Northeast CT tonight, and there have been some great songs sung about the blue moon.
Check out these:
Nat King Cole, Blue Moon
Chris Isaak, Blue Moon
The Marcels, Blue Moon (this one I'm sure you've all heard - reminds me of American Graffiti and Happy Days...)
Dean Martin, Blue Moon (this guy kills me!)
Billie Holiday, Blue Moon (simply awesome call back to the old days)
Frank Sinatra, Blue Moon (I don't care what anyone says, I am a Frank fan)
Being a daughter of a Moone, I can surely appreciate a good blue moon! I am off to enjoy it now.
Hoping you have a clear sky to enjoy your views as well.