Translation:The moon is the roundest during Mid-Autumn Festival.
Why is 'the' required? At least in Australian English we do not use 'the' when referring to festivals/days religious observance (eg Christmas, Easter, New Year's Eve, Diwali).
Counter examples: the summer solstice, the winter solstice, the vernal equinox. In the UK at least, Diwali is also often called the festival of lights.
"fullest" is better than "roundest". The "full" moon captures best the essence of the intended meaning but is not a literal translation.
Roundest = most circular.
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.