Blue Moon

A Blue moon is either astronomical event or used in a phrase as a colloquialism.

Astronomical Event
A Blue Moon occurs when a month has two full moons in it or four full moons within three months.

Colloquialism
Traditionally, the colloquialism Blue Moon means an unusual and not very frequent occurence.

In the World of Nausicaä
A Blue Moon seems to be a colloquialism, based upon the sentence structure used {{sup|.