What would you like to know?

What do you recommend.

If the goal is to have a tall fescue lawn without crabgrass: killing crabgrass before it produces more seed, seeding desired cultivar at proper time, and suppressing emergence of new crabgrass are all important. Your location remains a mystery, yet it is important in guiding what to do when.

zone 7a, Is fall a good tie to kill the crab grass? what killer do you recommemd. Thanks

7A is still a mystery. :-) What's your city and state?

Yes, any time is a good time to kill crabgrass, and you probably have a northern lawn in 7A. Probably. If that's true, then use Weed B Gone Crabgrass, etc. to get rid of it. If it's really widespread, then blanket spray. Two weeks or so later, spot spray whatever remains.

Reseeding during that is problematic. As in, the herbicide won't allow it, and reseeding time is right now. So I might be inclined to do it once, then reseed on Labor Day or so. That's plenty of time for fescue. There's tons of threads on how to reseed properly, but ask if you need more info,

Next spring, use any good pre-emergent to suppress crabgrass during summer (CG sprouts when soil temperatures get rather warm, but other weeds start when the forsythia blossom--that's a poor indicator, but the best we've got unless you can get a good soil temperature indicator). Drop then for most weeds, and keep the barrier up for crabgrass for summer, as it prefers to sprout when temperatures are much warmer.

And that should certainly be the end of the major problem.

If the lawn is largely crabgrass now, with little to no desirable turf, an option is to use glyphosate to kill it, thus allowing reseeding now, at optimal time time, w/o any worry about suppression of germination or subsequent growth of new lawn.

7A you guys! He's obviously in Alaska on one of the Aleutians.

Did he reseed in the spring? That's why he has crabgrass. Fall is the time to reseed cool season grasses.