Monncakes are in the shape of a full moon on the mid autumn

是应该用on。如果没有修饰成分,通常说in the morning/ afternoon/ evening,或者at night但是如果morning等词前面有修饰成分,即特指的某个早晨、下午或晚上等,通常用介词on。如。：on the morning of July 2； on Sunday afternoon； on a cold winter evening等。