I mean usually the first 2-3 days are the toughest. Most people begin to feel lighter after that. But if you’re not used to fasting you may also start feeling tired at any point. I’d say try out your schedule and see how it works for you. This way you’d be taking your test 4 days into the fast. But if fasting for that long is difficult you can also fast for two days, eat a meal, and then fast for two more days
Nobody can tell you.
Fasting is a very YMMV kind of thing. My hardest day is always the second day (I’m already keto adapted). I don’t advise learning the ropes of fasting when you have very important things going on in your life as your body is adjusting and you are learning how your body does the best