Why should the Crushing of Concrete Cube or Cylinder Samples be done at 28 days?

It is to check compressive strength of concrete.

Crushing of Concrete cube or cylinder samples be done at 28 days for to know the strength of casted cube at 28 th day. after we will compare with design strength (required) and actual strenth at site 

Because the concrete takes it about 28 days to each close to its full strength and hardness.