Once in a Blue Moon
Have you ever seen a blue moon? When and where? Blue moons are best known from the phrase “once in a blue moon,” which means “extremely rarely.” The first recorded use of this idiomatic phrase is in an anti-clerical flyer in 1528, published by William Roy and Jeremy Barlowe. In reference to the clerical corruptions, one said “O churche men are wyly foxes […] Yf they say the mone is blewe / We must beleve that it is true / Admittynge their interpretacion.” The context is not one hundred percent clear, but a number of websites interpret this as a reference to priests who required laymen to believe in their statements regardless of how false or ridiculous these were.
A current example would be: “Once in a blue moon I go to a concert, but only when there is a singer I really like.”