September is the best month to rejuvenate cool-season lawns (tall fescue and/or Kentucky bluegrass) that have been beaten down by summer heat stress, disease occurrence and weed invasion. Nitrogen fertilizer should be put down at a rate of 1 lb N per 1000 sq ft around mid- September, and if the lawn isn't as dense as you'd prefer, overseeding should be conducted at a rate of 4-6 lbs of pure live seed per 1000 sq ft.
When applying any product (fertilizer, pesticide or seed) to the lawn, the applicator should understand how to do it correctly. Too little, and the result may not be as desired. Too much is a waste of money, potentially hazardous to the environment or yourself, and illegal. The short video above along with the MU fertilizer calculator web application (http://agebb.missouri.edu/fertcalc/) should help you accurately apply the necessary ingredients for a successful green lawn.
REVISED: August 24, 2020