once in a blue moon

from The Collaborative International Dictionary of English v.0.48

blue moon \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]

   {once in a blue moon} very rarely; -- from the observation
      that the moon rarely has a bluish tint.

   Note: 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 blue moon, a long
         time, used primarily in the phrase once in a blue moon,
         has become widely accepted.
         [PJC]