There are thirteen canonical full moons each year, although some years will have only twelve, because a synodic lunar month is more than twenty-eight days long (actually about 29½ 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. The original meaning of blue moon was the third full moon in a season when there were four Full Moons in that season.
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, this is the first day of the month, called Rosh Chodesh, and is still observed by some as a holiday for women.
The term esbat is probably a recent adoption, dating to the writings of Margaret Murray. It is derived from French esbat (modern ébat), meaning roughly "frolic, romp", with some sexual connotations. This term was used during the European witch trials to describe the supposed behaviour of witches engaging in Devil worship, and it has been claimed that Murray was misled by the word's coincidental resemblance to the word sabbat.