blue moon

1821 as a specific term in the sense "very rarely," perhaps suggesting something that, in fact, never happens (cf. at the Greek calends, and the native in the reign of Queen Dick and Saint Geoffrey's Day "Never, there being no saint of that name," reported in Grose (1788)); suggested earliest in this couplet from 1528:

Yf they say the mone is blewe,
We must beleve that it is true.