Once in a blue moon
Once in a blue moon
Meanings
Very rarely
Examples
I spent a lot of weekends in the Big Apple and I loved it, but now I live too far away and I only go there once in the blue moon 
Ho passato molti fine settimana a Manhattan e mi piaceva da matti, ma ora abito troppo lontano e ci vado solo ogni morte di papaMy aunt Theresa won the lottery twice in a row. She must be extremely lucky because they say it only happens once in a blue moon
Mia zia Teresa ha vinto al lotto due volte di seguito. Deve essere molto fortunata perché dicono che succeda ogni morte di papaThose distant cousins of mine are just plain boring, luckily we meet once in a blue moon and in those rare occasions we manage to put on a good face
Quei miei lontani cugini sono di una noia mortale, per fortuna ci vediamo ogni morte di papa e in quelle rare occasioni riusciamo a fare buon viso a cattivo giocoI have a so-called "friend" who doesn't live far from me, but I never see her. She shows up once in a blue moon and when she does, it's because she wants something from me
Ho una cosiddetta "amica" che non vive lontano, ma non la vedo mai. Viene a trovarmi ogni morte di papa e normalmente è perché vuole qualcosa da meOrigin
In 1528 the following saying appeared in print:
Yf they saye the mone is belewe,
We must beleve that it is true.
From which we understand that the original expression "once in a blue moon" meant "never". The change to its present meaning - hardly ever - seems to have occurred as, in very particular atmospheric conditions, the moon may appear to have a slight blue tint.
Another explanation is the following: two full moons in the same month are extremely rare, though they do happen. A second full moon has come to be called a blue moon. This is apparently because the Maine Farmers Almanac used to list the date of first moon in red text, and the second moon in blue