Days of the Week
Every day at dawn, a new plane appears in the Kirinal Pit. This plane, the Manifest can be partially controlled by the Navigator of Worlds using the Orrery of Worlds. Before the methods for controlling the Pit were created, the mortal races hunkered in their fortresses, subject to unknown planes randomly appearing for indeterminate amounts of time.
The taming of the Pit and the introduction of a little order has led to the greatest flowering of civilization and magitech in the history of the world. Great fortresses and cities have grown around the Pit, driven by the influx of extraplanar resources that they can plan for and harvest.