Translation:The moon is the roundest during Mid-Autumn Festival.
I can't believe how inconsistent the whole course is. The previous level of festivals exercise wouldn't accept 'the' in front of the Mid autumn festival and the Spring festival. Now it doesn't accept answers without THE. My suggestion: while you are trying to figure out who is the smartestest and most Englishest of all the contributors, people are constantly frustrated by the English and so many inconsistencies in this course. Just forget the whole thing about The and A too. Accept both versions everywhere because the vast majority of people is here to learn Chinese. If someone wants to test proper use of THE they can just take the English course, problem solved.
I don't understand what this sentence means. There are at least 12 times a year when the Moon appears to be equally full or "round", not only during the Mid-Autumn Festival. Perhaps this sentence is trying to say that the full moon appears to be largest during the Mid-Autumn Festival (not true: a super moon, the moon is at its closest approach while orbiting of the Earth, can happen almost any month of the year).