The origin of the term "blue moon" is steeped in folklore, and its meaning has changed and acquired new nuances over time. Some folktales say that when there is a full blue moon, the moon has a face and talks to those in its light.
Historically, moons were given folk names, twelve each year, to help people to prepare for the related weather and crop needs. Names varied with locality and culture, often with descriptive names such as harvest moon, growing moon, snow moon, and egg moon. Most years have 12 moons (giving 12 names), but in the years with thirteen full moons the monthly "seasons" would be expected to come too early – for example, hens would not recommence laying their eggs by the fourth full moon since it was still too cold – so the early moon was named a "blue moon". This then re-aligned the rest of the year's moons and "seasons".
The last blue moon to shine on New Year's Eve was 19 years ago, in 1990.