A mind is like a door.

Father William Devon (32) - The priest of the local Catholic church and known to be very liberal in his views, William has managed to increase youth attendance at church without alienating the older generation and is known for having a very open mind. As Pastor Michael James (49) of the Anglican church said about him, "An open mind is fine, but a mind is like a door: if you keep it open you might as well not have one." They don't get along.

But I'm fairly sure that that's not where I originally read the phrase.
It's wrong about Blue Moons. Everyone's wrong about Blue Moons. It's got nothing to do with farmer's almanacks. It goes back to the seventeenth century, at least, and it doesn't mean "occasionally, maybe every couple of years", it means "pretty much never".
I wonder why so many people are mis-informed?

I guess it like the common law wife thing, some one hears it passes it on and suddenly it is Gospel. The number of people I have had to dispel this particular 'fact' to is uncountable.

I have always used the blue moon to mean pretty much never.

So where does it originate from and why?
Someone said so in the web somewhere, I guess.

So where does it originate from and why?

IIRC the oldest version was something like "He'd say the moon was blue" - now we'd say "He'd say black was white". Later "When there's a blue moon" or something similar would be like "When hell freezes over." Then "Once in a blue moon" meaning Never. BY the 20th century it had softened a bit and just meant almost never - the sort of thing you might see once in a lifetime if you were lucky.

The web definition doesn't work, of course, because there are two full moons in a month every year or two, and once in a blue moon is far less often than that.