In the IMO, the idea is that the first day you get P1, P2 and P3, and the second day you get P4, P5 and P6. Usually, ordered by difficulty, they are P1, P4, P2, P5, P3, P6. So, usually P1 is "easy" and P6 is very hard. At least that is the intended order, but sometime reality disagree.
Edit: Fixed P4 -> P3. Thanks.