Once in a Blue Moon

Any feedback is welcome!! (HINT HINT) 
My understanding is that this phrase means that it only happens once in a blue moon. Or never. Haha. It's like saying, I'll do that when pigs fly or something. It will just never happen. So, though I know the basic meaning of this phrase, I had no idea what the origin was. So, I hope you all enjoy!
According to the popular definition, it is the second Full Moon to occur in a single calendar month.
The average interval between Full Moons is about 29.5 days, whilst the length of an average month is roughly 30.5 days. This makes it very unlikely that any given month will contain two Full Moons, though it does sometimes happen.
On average, there will be 41 months that have two Full Moons in every century, so you could say that once in a Blue Moon actually means once every two-and-a-half years.
http://www.obliquity.com/astro/bluemoon.html
he earliest recorded English usage of the term "blue moon" was in a 1528 pamphlet violently attacking the English clergy, entitled "Rede Me and Be Not Wrothe" ("Read me and be not angry"; or possibly "Counsel Me and Be Not Angry" ): "Yf they say the mone is belewe / We must believe that it is true" [If they say the moon is blue, we must believe that it is true].
Some interpret this "blue moon" as relating to absurdities and impossibilities, and a similar moon-related adage was first recorded in the following year: "They would make men beleue ... that þe Moone is made of grene chese" [They would make men believe ... that the moon is made of green cheese].
An alternative interpretation uses another Middle English meaning of belewe, which (besides "blue") can mean "betray". The church was responsible for the calendar, and used the complex computus to calculate the important date of Easter based on the full moon. The late winter time before Easter is the Lent period of fasting. Easter falls on the first Sunday after the first full moon of Spring, where the beginning of Spring is fixed at 21 March in the calendar. By the 16th century, before the Gregorian calendar reform, the medieval computus was out of sync with the actual seasons and the real Moon. Occasionally, Spring would have begun and a full moon passed, a month before the computus put the first Spring moon. The clergy would have to tell people whether the full moon was the Easter moon or a false one, which they may have called a "betrayer moon"; after which people would have to continue fasting for another month.
http://en.wikipedia.org/wiki/Blue_moon
I thought that was very interesting!! Are you guys finding this theme interesting and fun, or do you think I should move on? I don't want to bore you all. Also, if there's a particular phrase you would like me to write about, just let me know! 
So, I'm gonna go thrift store shopping today. I need some props for the play, and hopefully I can also find the rest of my costume. So, there may be a post about my adventures in thrift stores later this weekend. :) I've never ever been thrift store shopping, so I'm looking forward to it!
Hebrews 12:1-2 "Therefore we also, since we are surrounded by so great a cloud of witnesses, let us lay aside every weight, and the sin which so easily ensnares us, and let us run with endurance the race that is set before us, looking unto Jesus, the author and finisher of our faith, who for the joy that was set before Him endured the cross, despising the shame, and has sat down at the right hand of the throne of God."