The world of Pellatarrum is a disk of infinite size with a spindle rising from its center, like a child's toy top. (The technical term would be "gyroscopic rotor".) This spindle is colloquially known as the Dayspire, because it is responsible for providing Pellatarrum with a day/night cycle.
There are no moons, stars, or suns in the sky above Pellatarrum. Instead, light and darkness are handled by the Positive and Negative Energy Planes, respectively, who are perpendicular to Pellatarrum's disk and are equidistant from it. The Dayspire is precisely large enough to eclipse the two energy planes.
Behold my crappy art. I spent literally minutes on this thing, just for you.
As Pellatarrum rotates upon its spindle, different parts of the disk are exposed to positive or negative energy, or a combination of both. This gives Pellatarrum a cycle of 6 hours of dawn, 6 hours of daylight, 6 hours of twilight and 6 hours of night. A later post will explore in greater detail the ramifications of this cycle.
A note for the scientific-minded: In this cosmology, darkness and cold are things which can be radiated, as opposed to merely being the lack of light and heat.
Pellatarrum slowly rotates within its own pocket universe, which is a prolate spheroid, or what we Americans would call a football. To continue the metaphor, the energy planes would be the tips of the football, stopping at the white painted rings, and the air inside it would be the Astral Plane.
The four panels of the football (sans tips) would be the four Elemental Planes of Air, Fire, Water, and Earth. Each element is created by the Positive Energy Plane and processes towards its destruction at the hands of the Negative. As an example, Water would begin its existence as Steam, coalesces into Water proper, and then turns to Salt shortly before destruction.
Seasons are the result of the Elemental Planes rotating about the football's axis. Spring is Air, summer is Fire, fall is Water and winter is Earth. Yes, the elemental plane of the season actually occupies the sky above Pellaterrum. Yes, during the summer, fire is known to fall to the ground. Yes, it falls through an infinite void and yet somehow strikes the ground of another plane. Because FANTASY, that's why.
Another post will cover seasons in more detail. This has just been a brief overview so that I can blow your minds now and clear up any confusion you may have before I delve into the trippy ramifications of this universe.