This is a bit late, but I think you're confusing necessary with sufficient. Instead of it being phrased as /August --> Holiday, which means that if it isn't August, then there must be a holiday, it should be August --> /Holiday (if it is August, there is no holiday). There could be other months without holidays, such as September or June, but your way of phrasing it means that there MUST be a holiday if the month is not August. This is switching the necessary with sufficient. The answer just states that if it is August, then there is no holiday, keeping the assumptions contained. Hope this makes sense!

I think of it as "poking holes" in the conclusion: if you can make any reasonable assumptions/questions that call into question the foundation of the conclusion, that makes the argument weaker. The more reasonable assumptions/questions you can think of, the weaker the argument. Ex. could another pet in the home have done this? What if Mr. Fat Cat doesn't like fish? What if Mr. Fat Cat had eaten something else? What if Mr. Fat Cat cleans his face like that also when he hasn't eaten? etc. You're looking for any gaps that point to info they didn't include that would have an impact on the argument. Hope that helps!