(This is for the fanfiction "Crossover Together". Info may change from canon information)
The Shinigami Realm is a dry and arid realm. According to Winry Rockbell, it is a realm that no Demon or Angel, no matter how high in rank, wants to venture into willingly, and is where the worst go to when they die.
Geographically wise, the realm does indeed follow night and day, but at night it is impossible to see without aid of physical light such as a torch, as there is no stars or moon. Days in the realm usually present a white haze in the landscape.