origin: biblical, a "false" moon (re: Lent); coined specifically as a phrase (1815–25)
1. a full moon occurring twice within a calendar month
→ such as August 31 (this coming Friday)
→ next opportunity won't be until July 2015
2. may also refer to the 3rd full moon in a season with four full moons.
3. extremely rare event of an actual bluish cast, due to volcanic ash (ex: Krakatoa, 1883).
4. informal; "once in a blue moon", meaning it happens very rarely or almost never.
→ modern interpretation; relates to absurdities and impossibilities