As soon as you see new growth. But please be careful and follow the instructions. That stuff is poison and can kill a lot more than weeds. Maybe you can look into more organic ways to killing the weeds.
You want to put down some weed seed emergent stopper like corn gluten meal when forythias start to bloom. Most Spring-germinating weeds start at the same time. There are always some forsythias in the neighborhood, so that is a good timing signal.
The regular "Turf Builder" Weed and Feed is a post-emergent and doesn't control crabgrass, or do you mean their specific "Crabgrass Preventer with lawn food"?
