Everybody confuses pre-emergent and post emergent weed killers. People are in stores looking for "weed-n-feed", when what they really want is a weed preventer.  A pre-emergent helps keep weeds from starting, a post emergent (like 2-4-D), kills weeds that are already growing.

Pre-emergent products prevent seeds from germinating, or inhibit the growth of the seeds, and therefore they die.  It's important to remember that the product can't tell the difference between a weed seed and grass seed. Don't waste your money on grass seed if you have applied a pre-emergent weed killer!

In my opinion, the best time to put down pre-emergent weed killers is April 1st, followed by an additional application on July 1st. The pre-emergent works by creating a barrier. It is strongest after watering-in on the first day it is applied, then deteriorates over time.  The first application takes care of April - May, and the 2nd application takes care of July- September.  By October, it's usually too cold for seed to germinate. That's why I think the commercials I see while watching baseball in September encouraging people to apply a pre-emergent that late in the season are wrong.