once in a blue moon

💡 Meaning

Something that happens very rarely.

아주 드물게, 좀처럼 일어나지 않는 일을 가리킬 때 써요.

🧠 Memory Hook

A "blue moon" is the second full moon in a single calendar month — a rare astronomical event. Seeing two full moons in one month is so uncommon that it became the symbol for anything that rarely happens.