Postulates of Statistical Mechanics
We are now in a position to define the two postulates of statistical mechanics. They are

1. All microstates of the system having the same energy and the number of molecules have equal probability ( also known as the equal a priori probability principle)
2. The infinite time average of any thermodynamic property in a macroscopic system is equal to the average value of that property over all the microscopic states of the system (also known as the ergodic hypothesis). In such a condition, each individual state (resembling the thermodynamic state of the actual system)  is weighted with the  probability of occurrence.

The second postulate merely strengthens the fact that a experimental measurement is a longtime measurement on a molecular time scale. Thus in a effort to measure the time between various miscrostates, the assembly of molecules will undergo a large, statistically representative number of microstates. So we are in a position to replace time average with statistical average. Thus the two postulates defines the scope and framework of statistical mechanics. As can be seen from their definition that the first postulate directs us in selecting the correct probability distribution, while the second one computes this probability distribution which will be equivalent to the property we are going to measure via experiments.