Black moon

The term black moon refers to an additional new moon that appears in a month or in a season. It may also refer to the absence of a full moon or of a new moon in a month.
Definitions, frequency, dates.
Month with two new moons.
One use of the term is for the occurrence of a second new moon in a calendar month. This is analogous to the by-month definition of a blue moon as the second full moon in a month. February is too short for a second new moon to occur. This event occurs about every 29 months.
The assignment of a calendar date to a new moon, and in which month a second new moon occurs, depends on the time zone. For example, the new moon of 2016-10-01T00:11 UTC occurs on 1 October for Europe, Asia and Oceania, making it the first of two new moons in October. However, for the Americas the date is still 30 September, making this the second new moon of September.
Calculated in UTC, instances of a second new moon in a calendar month between 2010 and 2020 are:
Season with four new moons.
Another use of the term is for the third new moon in a season that has four new moons. This is analogous to the "Farmers' Almanac" definition of a blue moon as the third full moon in a season with four full moons. A season lasts about three months and usually has three new moons. This event occurs about every 33 months.
There is no dependency on time zones in this definition as the seasons are tied to the winter solstice. Instances of four new moons in a season are:
Month without full moon.
Another use of the term is for the absence of the full moon from a calendar month. This can occur only in February; it happens about every 19 years. When February is without full moon, then the preceding January or December and the following March or April have two full moons.
As with the case of two new moons in a month, whether a black moon by this definition occurs depends on the time zone. Calculated in UTC, instances of a month without full moon between 1990 and 2040 are:
Month without new moon.
Another use of the term is for the absence of the new moon in a calendar month. This can occur only in February; it happens about every 19 years. When February is without new moon, then the preceding January or December and the following March or April have two new moons.
As with the case of two new moons in a month, whether a black moon by this definition occurs depends on the time zone. Calculated in UTC, instances of a month without new moon between 1990 and 2040 are:
Paganism.
In some aspects of paganism, particularly amongst Wiccans, the black moon is considered to be a special time when any rituals, spells, or other workings are considered to be more powerful and effective. Others believe rituals or workings should not be conducted at these times.