pre-emergent weed killer
You can apply the pre emergent early spring but can't reseed for a minimum of 6 weeks because the pre emergent will block germination for that long, as would other weed control products as well. You will want to get your lawn raked as clean as possible before applying the pre emergent. Then leave it alone. This because the pre emergent sets up a chemical barrier on the soil surface and any disturbance would compromise that chemical barrier, a big reason why pre emergents may seem not to work sometimes. Crabgrass is a hot weather plant and can come on after the pre emergent effectiveness may dwindle if put down to early in the spring. When forsythias are in full bloom is a good gauge for getting it down at the right time.
End of August, early September is the ideal time to reseed, when days are shorter and nights cooler, which also gives the new grass time to become established before winter. It is also a time when crab grass is waning from less sunlight and heat. Use a starter fertilizer at reseed time and keep the new seed moist if possible.