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Let's start with an example: 

i = 0;
x = x++;

Will this make x = 0 or x = 1?

We all know the precedence rule and what post increment does (returns the value and then increment the operand). So , lets apply that: 

x = (x++);

x = 0; and x will be incremented. 

But the problem starts.

In any text book you would have read till this. But when will x be incremented? Is it immediate? Is it before the next operation is done?

The increment is not done immediately. C specifies certain points before which this (these kinds of side-effects) will be completed. As normally expected all operators in C doesn't form this points (as in the case of Java) and the obvious reason is to make C code run faster. And these points are called "Sequence points" in C.