The Warhammer Fantasy equivalent to Halloween. Unlike Halloween which is based on the calendar day of October 31st, Geheimnisnacht is when both the Warhammer moons of Mannslieb and Morrslieb are full and Morrslieb is the closest to the Warhammer World all year, boosting magic and thinning the barrier between the planes of reality. So basically its when everything bad happens. Although the calendar day rotates, it always takes place in the winter and the Elves set their solstice by it. The only other time of the year that the two moons are full is Hexensnacht, the Warhammer New Years Eve.
In other words, Geheimnnisnacht is when all the fucked up shit goes down. A not insubstantial amount of the Warhammer fiction takes place on that night, and most Army Books mention it at least once as a time when a battle happened.
Events to happen on Gehemnisnnacht include: