A knit dress at midi length is comfortable enough to wear for hours while still looking considered and formal. Knit fabric stretches slightly which matters when you are sitting through services and receptions. The midi length adds formality that a shorter hemline would not have. Black knit holds its shape through a long day without the structured feel of stiffer fabrics. Look for dresses with a defined waist or seaming through the bodice because knit can read as shapeless without intentional cut. The fabric should be thick enough that it does not cling in an unflattering way. These are the midi knit dresses that solve the problem of looking appropriate while being able to actually move and breathe.