August 31 – All night long
The term Blue Moon can refer to the second full moon in a given month. When
people say "once in a blue moon" they are speaking to the rarity of
the occurrence of a second moon happening in the same given month. Thus the
next scheduled blue moon – second full moon of a calendar month – will be
August 31, 2012.
Can there be two blue moons in a single calendar year? Yes. It last happened in
1999. There were two full moons in January and two full moons in March and no
full moon in February. So both January and March had Blue Moons.
next year of double blue moons is coming up in 2018.