Day and Night on the Disk

The material plane doesn't have a Sun or a Moon- its illumination is determined by which direction it faces in the inner sphere of the planes. Generally the Positive Energy Plane is throught of as "up" and the Negative Energy Plane is "down". Thus it is midday when the disk of the material plane is directly facing up and midnight when it is facing down. There are no astonomical objects in the sky, The Myst would obscure them even if there were, but there is a gradient in the sky enough to make it reasonably easy to determine midday and the cardinal directions by watching the sky for an hour or so.