What does "once in a blue moon" mean — and why is it funny?

informal

Meaning

Very rarely; almost never.

Where it comes from

A 'blue moon' is a rare extra full moon in a season or month. The moon does not actually turn blue, though smoke and ash have occasionally tinted it.

Why it is funny

The mild humor is that it sounds poetic and precise yet describes something most people have never knowingly seen. It dresses up plain old 'hardly ever' in a beautiful, faintly unreal image.

Used in a sentence

"He only calls his brother once in a blue moon."