Curse Day (14th Mumberbur) is the anniversary of the death of Lord Dimwit Flathead the Excessive in 779 GUE.
In that day, the great wizard Megaboz cast a mighty Curse which killed the Twelve Flatheads and eventually, in 883 GUE, brought about the collapse of the Empire and the destruction of Flatheadia.
The Curse Day of 883 GUE marked the last day of the reign of Wurb Flathead, and the beginning of the influence of the first Dungeon Master.