once in a blue moon 出处是哪里？

once in a blue moon
[英][wʌns in ə blu: mu:n][美][wʌns ɪn e blu mun]
adv.千载难逢地; 俟河之清; 百年不遇;

例句:

1.
If you do it once in a blue moon, employees may be wary.
如果你只是偶尔与员工接触一次，他们可能会放不开。

2.
It is only once in a blue moon that you get an opportunity.
你居然得到那么好的一个机会,真是千载难逢。

Once in a blue moon，它的意思是很难得到，几乎不可能。
也有一首歌:
<Once In A Blue Moon>
歌手：Lighthouse Family
Wish we could choose our own landscape,
Wish me and you could just go anywhere we wanted.
Once in a blue moon we'd stay out late
And watch the sun come up at five-thirty in the morning,
Oh, yeah, once in a blue moon.
(Chorus)
Day will surely come,
One day I'll be with you
Day will surely come
We will make it all come true
Day will surely come.
I ain't seen you in a month of Sundays,
I never knew exactly what it was I wanted,
Once in a blue moon y