'''Day and night:'''
The Earth rotates on an axis. When it rotates, half of the Earth is exposed to the sun and half is not. When a part of the earth is exposed to the sun it is ''day'' and when it is not exposed to the sun it is ''night.'' Pretty simple. 

'''The seasons:'''
axis------> / 
Earth--> (/)-------O<--Sun 
axis----> / 

Earth's axis is tilted so in summer (for the Southern Hemisphere) the Southern Hemisphere is in the sun longer than the Northern Hemisphere. 
The days and nights occur due to rotation.
due to rotation of earth around its own axis from west to east in anti-clockwise direction .