in NE crab grass preventer can be applied mid-May (unless unseasonably warm Spring
), the ground temp needs to be 65d for the seeds to germinate. I have been doing it that way for 30 yrs and no problems with crab grass. The chemical only last in the ground for 90 days so IF you apply too early can have problems with late season crab grass. MOST IMPORTANT: this is a physical chemical barrier that can be broken by activities such as raking the lawn, so do not disturb after applying.
I plan on putting down my 2nd bag of Scotts Winterizer this week with snow finally being gone. I buy 2 bags in the Fall, 1 goes down in Oct/Nov, 2nd in the Spring. You want to stimulate root growth this early season not greem blades, yet. Lawn will better survive dry summer with strong roots.