With a modern literal meaning of "midnight," the term witching hour refers to the time of night when creatures such as witches, demons, and ghosts are thought to appear and to be at their most powerful and black magic to be most effective. Adversely, the term devil's hour applies to the hour of 3am or 3:15; the suggestion being that this demonic hour is an inversion of the time at which Christ died at Calvary. The suggestion being that this hour is a mocking of the holy trinity.
It may be used to refer to any arbitrary time of bad luck or in which something bad has a greater likelihood to occur (e.g., a baby crying, or a computer crashing, or stock market volatility, crimes, supernatural, etc.). The witching hour from medieval times is the time believed that witches came out to do their "unholy" practices. Women caught out late at night could have been suspected of witchcraft if they did not have a "legitimate" reason to be out.