Post-emergent herbicides are designed for situations where you ALREADY have weeds present. Typically, this includes common spring or fall weeds; fall is actually one of the best times to effectively kill most weeds! Keep in mind, most post-emergent herbicides will not kill crabgrass or similar grassy weeds, because most post-emergent herbicides are designed to NOT kill your lawn (see label!). If you failed to apply a pre-emergent herbicide in the spring you will want to use a product containing "quinclorac" or similar herbicide formulation.
Remember, always read and follow the label
TIP: Generally, avoid spraying over 80º and don't overuse herbicides as some CAN damage your lawn.