A blue moon (using the Gregorian calender definition as opposed to the Farmers Almanac definition) occurs roughly every 2.5 years in any given month and occurs on NYE once every 19 years!!!
Here's some info on how the term came into common parlance:
Sky and Telescope calendrical misinterpretation
The March 1946 Sky and Telescope article "Once in a Blue Moon" by James Hugh Pruett misinterpreted the 1937 Maine Farmers' Almanac. "Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon." Widespread adoption of the definition of a "blue moon" as the second full moon in a month followed its use on the popular radio program StarDate on January 31, 1980.What the Farmer's Almanac really said was:
Farmers' Almanac blue moons
In the 19th and early 20th centuries, the Maine Farmers' Almanac listed blue moon dates for farmers. These correspond to the third full moon in a quarter of the year when there were four full moons (normally a quarter year has three full moons).The power of the media I guess. Anyway, Happy New Year to all my readers!!!