Apply pre-emergent when the forsythia blooms. Follow directions on the pre-emergent; it will let you know when it can be reapplied. Most fertilization for cool season grasses (KBG, fescue, rye) should be done in the fall. Avoid fertilizing in early spring; late spring, like around Memorial Day, is okay. Don't fertilize in the summer.

Can you recommend a good pre-emergent product?
I've tried scott's before but there are so many different brands that I hear mixed reviews about.

more specifically, is there a pre-emergent I can use to tackle broadleaf weeds and grassy kind of weeds?

I've noticed that there are Dimension products that can be mixed with water and granular form.
Is there a difference? Does one work better than the other?

Here is where I found the liquid form: http://www.dowagro.com/turf/products/herbicides/dimension.htm

Here is where I found granular form:
http://www.homedepot.com/p/LESCO-19-0-7-Dimension-Crabgrass-Preventer-080311/100185665

I use a pre-emergent without nitrogen and look for what's available at my local stores. Southern States had a granular product with Dimension (0-0-7) and I got that. I used something similar last year, but I may have gotten it from Home Depot then. If you plan to overseed in the fall, be sure to read instructions on how soon after using the pre-emergent you can reseed. That may affect which one you choose.

Scotts will do an excellent job, ... but a less expensive brand will, likely, do ALMOST as well ...

Thanks for all the info.
I've tried Scotts in the past and did not see great results.
I think ill try out Dimension this time.

Someone suggested to apply milogranite organic fertilizer with the Dimension.
Would this help create a more green and thicker lawn?
Or is it not necessary?

Whatever brand you get, ... you must apply BEFORE the crab grass germinates.

As the previous poster said .... "When the Forsythia blooms". If you wait until those blooms fall, ... you may have waited too late.

Think late March/early April ... one week window ...

Check your soil temperature. When it nears 50 F, apply pre-emergent and water it in unless it's going to rain soon. It needs at least 1/2" rain to activate. Lesco offers a Dimension product. If you need to control other grassy weeds, application timing is very critical. Too early an application and you won't catch them.
Soil Temp Chart

New Jersey Soil Temp