I want to know what is the event function name in which [[ Event gets triggered when BEFORE page or structure group is saved. ]]
I don't think that you should be punished for the question, especially if you are new to the Tridion SE. However, it is always helpful to give more explanation to the problem you are trying to solve. From what I think you are trying to achieve it trigger ES before save. For that you can use different event phases. In your case, I guess you can use Initiated phase, as per the following documentation:
The event handler is triggered just before the actual event. For example, just before saving a Component, you can trigger code that analyzes its contents, derives a set of Keywords, and adds these Keywords to the metadata of the Component.