The universe is vast, infinite, dotted with stars, planets and galaxies. Most of it is entirely unaccessible to any human-like intelligence; there may be other civilisations, other lives, out there, but we'll never know. What we do know about, however, is the Sphere.
The Sphere is perfectly round, perfectly smooth, perfectly immaterial, and perfectly terrible. It is a home, and a prison. It consists of nothing but a region of space where the laws of the universe have been bent and twisted. Mountains have risen on its surface, and water has formed a thin sheet over it; all human struggles play out near its threshold, on the surface of the fledgling world that encircles it.
All matter outside the Sphere is drawn towards its centre by an overwhelming force. All true matter inside is drawn towards the nearest edge. Were you to stand in its centre, at the eye of the heavenly storm, it would be equal in all directions, and you would be stuck in place or torn apart.
In other places in the universe, rounded worlds form by the strange and unknown force of gravity. In the vicinity of the sphere, gravity is like an insect in a stampede; it is drowned out, and the Sphere's own threshold force replaces it. Thus, our world did not form as a sphere, but rather as a shell around the Sphere, a number of flat continents that slowly spin in concert, dancing on the sphere's surface. Water spreads out through the threshold, thicker near the continents, but still just a sheet, making the Interiour hazy and murky. The Sphere itself moves, and its small blue sun moves with it, orbiting a common point in between. Thus, the day-night cycle; and thus the year.
Note that the above is written from the perspective of a relatively later scholar; earlier civilisations would have their own conceptions about the world, and possibly their own creation myths.
The Sphere is around seven thousand kilometres in diametre, making it slightly larger than Earth. It can move around, and it is susceptible to gravity as anything, over large distances. It forms a binary system with its small, hot blue star, with the system's centre about four-fifths of the way from the Sphere to the star.
The Sphere, as it has no features whatsoever and causes no friction with its movement, cannot spin, in that if it did, nobody would know and nothing would change. Thus, the continents spin on its surface. This creates both continental drift (They're not all at the same speed, but the differences are very minimal) and a day/night cycle.
The day is equivalent to 24 hours, although some cultures may use different measurements. The year has 365 days, although different cultures may have different calendars.