(CNN) -- It happens only once in a blue moon -- and scientists say a blue moon is exactly what we'll see in the skies this New Year's Eve. Don't expect an azure glow over our lunar satellite, however. The term "blue moon" simply refers to the second full moon in a calendar month, something that hasn't happened on a New Year's Eve for nearly 20 years, NASA says. "December 1990 ended with a blue moon, and many New Year's Eve parties were themed by the event," said Professor Philip Hiscock of the department of folklore at Memorial University of Newfoundland, in Canada. "It was a lot of fun." Most months have just one full moon, because the 29.5-day cycle of the moon matches up pretty well with the length of calendar months. Occasionally, there will be two full moons in a month, something that happens about every 2½ years, NASA says. But a blue moon on December 31 is rare. It is possible for the moon to have a cerulean hue...