Day and Night
Halg part of the earth is lighted by the sun at any point of time. The part which is facing the sun has day and the part which is not facing the sun has night.
It takes 24hrs for the Earth to do one rottaion. So in 24 hrs the whole gets earth day and night.

Example - Inida is at the middle of night so India has midnight then it moves and gets morning then the noon then eveneing and then night and then back to midnight.