The bad news is is that the best time apply crabgrass preventer is in the spring just as the forsythias are finishing up their flowering cycle (just as the lilacs are starting). There are summer crabgrass killers but they need to be applied to a wet lawn and not get watered off for a couple of days (like weed killer). Usually it takes a couple of applications for summer crabgrass killer to work. Even then, the crabgrass plants will probably already produced seed so you’ll need to apply a crabgrass preventer in spring anyway. I’d suggest that you apply crabgrass preventer for the next two to three spring to be sure you’re rid of it. Then monitor the lawn for the next couple of years since seeds may be redepostied onto your lawn by birds and such. Any digging may stir up crabgrass seeds from deeper in the soil to the surface where they’ll germinate into a new crop in that area. Crabgrass seeds buried too deep to germinate can remain viable for as long as 75 years!!!