Geheimnistag

This holiday does not fall on any particular day of the calendar; the Day of Mystery, also called the "Floating Holiday", occurs when the twin moons Morrsleib and Mannsleib are both full in the sky. This occurs only once per year and is considered a time when the veil between worlds becomes thin, sometimes dangerously so. The Day of Mystery is dedicated to Morr, and many strange and unique festivals occur on this holiday throughout the Empire. Auguries are commonly performed, sometimes with spectacular or terrifying results. Humans take this day to pay homage to their ancestors, and sometimes these spirits come back to converse with the living (see Ancestor Spirits).

Source

 * Warhammer Fantasy RPG 2nd ED -- Tome of Salvation -- pg. 145