Much like the rear delts on shoulder day, calves are often neglected—or saved for the last part of a leg workout, when you're tired and barely able to muster up the energy for a couple of half-assed sets.
As you probably already know, muscles can't grow if they receive subpar training. If you really want to make your calves stand-out, you must start training them the way you train your back or chest: fresh, from every angle, and to complete exhaustion.
If you can start your leg training with calves—and train them with the same intensity as you do your quads and hamstrings—do it. If you're trying to hit your upper legs super-hard and just don't have the energy to do the same to your calves on leg day, add an extra calf day into your split or add the work to a different workout.
Whatever you choose, the point is to ensure that your calves don't suffer from lack of attention.