The term blue moon has several meanings:
Most correctly, a blue moon refers to a full moon that is blue in colour. Under clear skies, this occurs when: 1) The planet Venus is close to Earth. Venus illuminates the full moon making it appear slightly pale blue in colour. 2) The moon can also appear blue if particles are in the sky from an volcanic eruption or by other means dirt is in the atmosphere.
A blue moon gives raise to the old expression, "once in a blue moon", meaning "it doesn't happen often".
In magic, a blue moon is very significant as it is considered an unusually good time to set specific goals in one's life.
In astrology, a blue moon refers to the second full moon while the sun has remained in the same sign of the Zodiac. Especially in the United States, a blue moon has come to mean a second full moon within a calendar month.
The blue moon has always been recognized as a special event and an especially romantic time. So, next time you see a blue moon think about your personal goals and that special someone in your life.