Once in a blue moon

Pro Tip 1/3

Idiomatic Expression

It's an idiomatic expression and not to be taken literally; there is no actual 'blue moon'.

My aunt comes to visit us once in a blue moon, maybe every few years.

Pro Tip 2/3

Rare Occurrence

'Once in a blue moon' refers to something that happens very seldom.

I travel abroad once in a blue moon; it's quite a special event each time.

Pro Tip 3/3

No Numeric Frequency

The phrase doesn't specify how often something happens, just that it's not frequent.

He might play football once in a blue moon, without any regular schedule.