There are thirteen canonical full moons each year, although some years will have only twelve, because a lunar month is more than twenty-eight days long (actually about 29 1/2 days). A "blue moon" is popularly defined as the second full moon in a calendar month, although some define it as the second full moon while the sun is in one sign of the Zodiac. Those definitions of a "blue moon" are of a recent date as the original definition that gave rise to the phrase "once in a blue moon" has been generally forgotten. The original definition related to the moon actually being blue. This occurs when Venus is very close to the Earth and geocentricly on the opposite side of the Moon from the Sun and the sky is very clear. The dark part of the Moon is then illuminated by Venus and appears to be a pale blue color.
In The Witches' Goddess, Janet and Stewart Farrar note that the Babylonians considered the new moon to be the time when the Goddess was menstruating, and it was bad luck to do work on that day. In Jewish culture, the new moon is the first day of the month, called Rosh Chodesh, and is still observed by some as a holiday for women.