blue moon

• POS: Noun Phrase

CIDE DICTIONARY

blue moon,  

 The second full moon occurring in the same month; -- derived from the expression once in a blue moon; as, we had a blue moon on January 31, 1999, and another in March.  [PJC]
"In 1980, the phrase blue moon was applied to the second full moon of any month, because this phenomenon is unusual. This new meaning for the old phrase blue moon has become widely accepted."  [PJC]

ROGET THESAURUS

blue moon

Diuturnity

N diuturnity, a long time, a length of time, an age, a century, an eternity, slowness, perpetuity, blue moon, coon's age, dog's age, durableness, durability, persistence, endlessness, lastingness, continuance, standing, permanence, survival, survivance, longevity, distance of time, protraction of time, prolongation of time, extension of time, delay, durable, lasting, of long duration, of long-standing, permanent, endless, chronic, long-standing, intransient, intransitive, intransmutable, persistent, lifelong, livelong, longeval, long-lived, macrobiotic, diuturnal, evergreen, perennial, sempervirent, sempervirid, unrelenting, unintermitting, unremitting, perpetual, lingering, protracted, prolonged, spun out, long-pending, long-winded, slow, long, for a long time, for an age, for ages, for ever so long, for many a long day, long ago, longo intervallo, all the day long, all the year round, the livelong day, as the day is long, morning noon and night, hour after hour, day after day, for good, permanently.