Once in a blue moon
Definition 1 of 1
Pro Tip 1/3Idiomatic 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 2/3Rare 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 3/3No 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.