It will be important to be sure you are dealing with crab grass. It is often confused with quack grass. The following explains some of the differences.
I will answer your questions assuming that you indeed have crab grass.
The following sites will provide information with respect to dealing with crabgrass:
In the above sites you will see reference to post-emergent herbicides. If you go this route, be sure to get a herbicide whose directions indicate that it is safe to use on regular grass but still kills crabgrass.
Crab grass sows its seeds in August and September. These seeds will then germinate in the following spring. Since crab grass can then appear year after year, it is easy to think of it as a perennial.
The pre-emergent herbicide should be applied as early as possible in the spring. The goal is to have this herbicide present before crab grass seeds germinate. A pre-emergent herbicide is to retard this germination.
The post-emergent herbicide could be applied now and extending into early September. The goal here is to kill the crab grass before it disperses its seeds.