once in a blue moon

idiom/wʌns ɪn ə bluː muːn/ 🔊 listen

Once in a blue moon means very rarely — so rarely it almost never happens. A 'blue moon' is the second full moon in a calendar month, a rare astronomical event, hence the idiom.

📖 Meanings

1very rarely, almost never
"She only calls me once in a blue moon." — She calls me very rarely.
2infrequently, at long intervals
"It rains here once in a blue moon." — It hardly ever rains here.

🔄 Related expressions

rarely →hardly ever →once in a while →

💬 Common patterns

once in a blue moonhappens once in a blue moononly once in a blue moon
❓ "He visits us once in a blue moon." What does this mean?