You have an existing stand of grass and want to plant new grass so 2 different issues exist -- how to fertilize the new seeds and also how to prevent crabgrass without preventing germination of your new seed. I used Scott's Starter w/crabgrass preventer (siduorn) and it works fine. A bit expensive....$45/5000 sq ft at regular home improvement center but my lawn is so small i don't care, that lasts me several seasons. By the 2nd week of April crabgrass is germinating in the area under normal conditions so if you wait much longer you risk crabgrass in your plot. My suggestion is to closely monitor the weather and search onlilne for soil temperatures near your major city i.e. I have 2 sites I follow and found simply by typing "chicago soil temperature" and there is a daily report. What you risk here is if you plant to early you may have your new seedlings killed off. Last year, because I am close to the lake I planted on Apr 3 and here by the lake it never froze although in the suburbs that don't have lake protection it would have frozen. A "safe" idea for you is this. 1st week of April, put down the starter fert with crabgrass preventer so that your plot is "immunized" from crabgrass...but don't seed the bare areas yet. So you are fully protected from crabgrass now, but the starter fert and preventer only lasts say 30 days. In mid-April (2nd or 3rd week), as soil temps come up, plant your plots that need new seed. The starter fert and crabgrass preventer will still be there and do their jobs. 30 days after your first app of the starter fert w/preventer (NOT 30 days after you plant), apply it again. Thus you have a new round of prevention and your new plot getting another shot of fert right after it comes up and is in need of another shot. Fescues in this area only take 2-3 weeks to come up, I did it last year with Supina bluegrass which takes a solid 3-4 weeks so you'll have enough time to get it rooted before it gets too hot.