Translation:What will you do the evening of Mid-Autumn Festival?
The point is we should be learning Chinese, not quibbling about "in" or "on" in English! They are way too inflexible with "correct" answers.
The exercise should also accept "On Mid-Autumn Festival night, what will you do?" Duolingo wants "the [sic] night of Mid-Autumn Festival what will you do?"
Shouldn't my answer also be acceptable? "what are you going to do on Mid-Autumn Festival evening"
"What are you all doing on the night of the Mid-Autumn Festival" should be accepted
"What will you guys do for the Mid-Autumn Festival night?" was rejected.
This answer makes sense but it sounds very unnatural. Perhaps "what are you guys doing mid-autumn festival eve?" or "What are you guys going to do for mid-autumn festival in the evening?"
"Eve" is incorrect, as the evening of Mid-Autumn Festival is the actual evening of the event, not the night before.