Wikipedia: "... blue moon is the "extra" full moon in years that have thirteen full moons. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. The extra moon is called a "blue moon." Different definitions place the "extra" moon at different times."
Generally, as in Terre Logsdon; "Blue moon to occur on New Year’s Eve"; Lake County News (Dec.30'09): "To be called a “blue moon,” the moon must be full for the second time in a calendar month, which will occur on Thursday, Dec. 31 – New Year’s Eve – for the first time in almost 20 years. A blue moon occurs every two and half years, but the last time there was a blue moon on New Year’s Eve was in 1990 – and the next will be in 2028."
