It could bring with it world destruction, black magic or the Second Coming - or just get moongazers out in force.
The last day of September will see a Black Moon rising, a rare lunar event which causes the sky to go dark and the moon become invisible.
It's a time associated with powerful spells and rituals in pagan traditions, with other beliefs considering it to signal the end of the world - or even the second coming of Christ.
The rare lunar event happens when the Earth experiences a second new moon in a month, though there are other less common forms of Black Moon.
Tomorrow's astronomical event is similar to a Blue Moon, but in reverse. A Blue Moon occurs when there are two full moon's in a month, a Black Moon appears when there are two new moons in a month.
Because it happens when the moon falls into the Earth's shadow, different parts of the world will see the black moon at different times.
This week, it can be observed in the skies of the Western Hemisphere.
In the UK, it will be at its height at about 1.10am on Saturday morning, but will reach its peak in the US at around 8pm on Friday.
Those on the other side of the world will see it next month, when it coincides spookily with Halloween.