Meaning of "once in a blue moon"

[Helmuth]

English Idioms and Expressions, Advanced Level

ESL/EFL Test #23 "Idioms with prepositions", question 4

We haven't seen him for ages but he turns up once in a blue moon.

(a) very rarely
(b) sometimes
(c) occasionally
(d) frequently

English Idioms and Expressions, Advanced Level

ESL/EFL Test #23 "Idioms with prepositions", answer 4

We haven't seen him for ages but he turns up very rarely.

Correct answer: (a) very rarely
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what do you mean by once in a blue moon?

Helmuth

[Pamela]

Hi,
Once in a blue moon is another way of saying very rarely(as in your test sentence)

http://www.phrases.org.uk/meanings/269500.html