en
blue moon
/bluː muːn/
Definition
1
The second full moon to occur within a single calendar month, a relatively rare astronomical event.
2
An idiom meaning something that happens very rarely or infrequently, as in 'once in a blue moon'.
Examples
I only clean my fridge once in a blue moon, so it's basically a science experiment in there.
Finding a genuine vintage vinyl at a garage sale happens once in a blue moon these days.
He apologizes sincerely once in a blue moon, usually right after his phone dies mid-argument.
Spotting a unicorn in traffic is about as likely as a blue moon on a Tuesday.