With warm season, you can apply the pre-emergent of your choice in September. If you missed that window, spray with Simazine in November.
Fescue is a little tougher because you normally overseed in the Fall. Therefore, you cannot apply weed control until at least 45 days after seeding. Once you apply the pre-emergent, your seed will stop germinating. Sometimes it is better to do a post control such as Trimec or Speedzone and hold the pre-emergent application until next Spring. Fescue seed tends to germinate for a long period of time.