Monkey grass, also called Liriope, should be cut back in the late winter or early spring. This is a time when the new growth buds are beginning to emerge, and it is optimal to remove the old, dead grass from the previous season before new growth begins in earnest. Be sure to remove any dead foliage and trimmings from the lawn after you have cut the grass back.
Doing this will help the grass recover sooner and become more lush and green.