Question

what is the meaning of idiom" once in a blue moon"

Answer 1

The idiom ‘once in a blue moon’ refers to as happens very rarely.

The main concept behind the origin of the idiom is second but rare occurrence of full moon in the similar month.

Here blue moon refers to the full moon which occurs second time in the same calendar & this phenomenon occurs only in 32 months.

Answer 2

It is used to refer things that happen very rarely.