once in a blue moon

[ wuhns in uh bloo moon ]

idiom

1. blue moon ( def 5 ).

once in a blue moon

1. To do something “once in a blue moon” is to do it very rarely: “That company puts on a good performance only once in a blue moon.” The phrase refers to the appearance of a second full moon within a calendar month, which actually happens about every thirty-two months.

Word History and Origins

Origin of once in a blue moon¹

First recorded in 1830–40

Idioms and Phrases

Rarely, once in a very long time, as in We only see our daughter once in a blue moon . This term is something of a misnomer, because an actual blue moon—that is, the appearance of a second full moon in the same calendar month—occurs every 32 months or so. Further, the moon can appear blue in color at any time, depending on weather conditions. [Early 1800s]

Related Words

• every now and then
• every so often
• infrequently
• intermittently
• now and again
• occasionally
• periodically
• sporadically