The Witching Hour is the hidden 13th hour between the end of one day and the beginning of the next. Normal humans can only enter it once a year: during the autumn when an hour is added to the day to move time back from daylight savings time to standard time. Otherwise, it can only be accessed by those knowledgeable of the arcane arts. Even other supernatural creatures can find the witching hour impenetrable without knowledge of chronomancy and how to properly slip between the days. During this hour, the moon hangs still and full in the sky, and the world is held motionless within a tide of dark matter.
During the witching hour, one effectively has an extra hour within the day to conduct whatever rituals you wish to. Within this hour, dark matter abounds and magic is incredibly easy to wield. However, any besouled creature not inside the witching hour itself is completely unaffected by anything performed within it. In effect, humans and other supernaturals just do not exist during the witching hour unless they enter it. Likewise, fire doesn't burn and electricity doesn't flow unless it is encouraged to do so by magic. In this way, one cannot start a fire to burn down a town within the witching hour, but one can keep a fire beneath a cauldron burning. This also makes electronics completely inoperable during the witching hour unless you're a technomancer--and even then, they can usual only activate a single device they can touch, meaning networks and the Internet itself are completely inaccessible. As such, it is usually just used for planning, scouting, setting up rituals, clandestine meetings, and the like.