Read this: Poland's successive kings granted privileges to the nobility at the time of their election to the throne (the privileges being specified in the king-elect's Pacta conventa) and at other times in exchange for ad hoc permission to raise an extraordinary tax or a pospolite ruszenie.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): When did kings grant privileges to the nobles?
at the time of their election to the throne