Once in a Blue Moon
“Once in a blue moon” means very rarely. The rare-time sense of “blue moon” is documented in Pierce Egan’s 1821 Real Life in London as “haven’t seen you this blue moon,” with a note explaining that it implied a long time; specialist archival research reports the exact “once in a blue moon” form in 1833.
The idiom means “very rarely.” Its rarity sense is older than the modern popular definition of a blue moon as a special full-moon occurrence: an 1821 text already uses “this blue moon” to mean a long time, and specialist archival research reports the exact “once in a blue moon” wording in 1833.
Key details
What the evidence supports
The rarity idiom comes from the modern definition of a blue moon as the second full moon in a calendar month.
The rarity sense is documented in the nineteenth century, long before the later twentieth-century calendar-month definition became widespread.
The exact phrase was already fixed by 1821.
The 1821 primary text has “this blue moon” with a long-time gloss, not the exact fixed phrase “once in a blue moon.”
Documented timeline
- 1821
“this blue moon” rarity/long-time sense
Real Life in London prints “this blue moon” and glosses “Blue moon” as implying a long time.
- 1833
Exact fixed wording reported
Specialist archival research reports an 1833 example of the exact wording “once in a blue moon.”
Further guidance
Documented Predecessor
In Real Life in London (1821), Pierce Egan prints “haven’t seen you this blue moon” and supplies a note explaining “Blue moon” as implying a long time.
Exact Phrase Chronology
Specialist archival research identifies an 1833 Athenæum example using the exact wording “once in a blue moon.” The engine records that as reported archival evidence rather than claiming it is the absolute first use.
Meaning
Cambridge gives the idiom the straightforward meaning “not very often” or “rarely.”
Origin Status
The nineteenth-century rarity usage predates the later popular calendrical explanation involving an extra full moon. The engine therefore does not derive the idiom from that modern calendar definition.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Once in a blue moon — Cambridge Dictionary (opens in a new tab)Cambridge University Press · Current meaning
- Real Life in London (1821) — “this blue moon” passage (opens in a new tab)Pierce Egan / digitized historical text · Primary text for the 1821 rare-time “this blue moon” usage and authorial gloss
- The authentic origin of “once in a blue moon” (opens in a new tab)Word Histories · Archival report of exact 1833 “once in a blue moon” wording