You don't use it before a good thing, in the metaphor the 'storm' represents something bad, violent or uncertain and 'the calm' something good, pleasant or tranquil. You don't normally use it after the bad thing has already happened. The common usage would be before something significant is to happen, where there is uncertainty and there is potential for things to go really badly wrong. For example, soldiers waiting quietly before they go into battle. It isn't certain that things will go wrong and they will be killed, but there is a possibility. So the quiet moments before they go to fight could be described as the 'calm before the storm.'

It's a quiet period beforesomething bad haqppens. Calm is good Storm is bad