A relatively uncommon celestial event will occur in the western hemisphere this Friday, 30 September 2016, when a so-called "black moon" rises in the sky. Don't strain your eyes trying to see it, though. "Black moon" is simply a name given to the second occurrence of a new moon in a single month. New moons are essentially invisible to the naked eye, because the sun is shining on the side facing away from earth.
A black moon occurs once every 32 months, the result of the discrepancy between the actual number of days in a lunar cycle (29.5) and the number of days in a calendar month (30 or 31). It's the obverse, if you will, of a blue moon, most commonly defined as the second occurrence of a full moon in a given month, which also occurs once every 32 months (thus the use of the phrase "once in a blue moon" to designate things that happen relatively infrequently).
It's important to note that neither of these names were coined by astronomers, who in fact only utter them when prodded to explain a sudden surge in their popularity on social media every two or three years. They originated in folklore.
Once upon a time, the phases of the moon were critically important in the affairs of human beings. Our ancestors reaped and sowed by them, navigated the seas by them, and relied on them to predict everything from the ebb and flow of the tides to menstrual cycles to the genders of unborn babies (or so it was believed). The convention of naming full moons arose as a memory aid for keeping track of the seasons, a helper we no longer need thanks to the invention of the calendar, though names like "harvest moon," "wolf moon," and "blood moon" still hold a certain charm.
Moon name memes are incredibly popular on Facebook and Twitter, especially when there's a supernatural tie-in, as in this example of a black moon meme circulating in September 2016 which warns that the world may be coming to an end:
And this one, also from September 2016, predicting an astrological "push in the right direction":
This is all codswallop from a scientific point of view, of course, but witches will want to take note of the fact that this is an "extremely powerful time for any magick workings."
... Or so we read on the Internet.