If you have a cool season (fescue) lawn, it may be looking a bit tired, weedy, or bare in spots by September. Fortunately, this is also the perfect time of year of overseed or even renovate your yard. The temperatures in September are generally ideal for fescue germination, and the new seedlings have many months to grow and develop a strong root system before the stresses of the next summer arrive.
First, control any large patches of weeds. Mow and remove clippings. If your lawn has excessive thatch or bare patches, rake. Core aeration is also beneficial, as it reduces compacted areas in the soil and provides more seed-to-soil contact. Fertilize at a rate of 1 lb nitrogen per 1000 feet. NCSU publishes lists of recommended fescue and Kentucky bluegrass cultivars for our area (see links for the 2013 recommendations). The seeding rate is generally 5-6 lbs of seed per 1000 feet. Keep your lawn moist until seedlings are established and mow at the recommended height of 3.5 inches.
If your lawn is very sparse or weedy and needs extensive renovation, please see this article by Durham Co. Extension Agent Michelle Wallace, published in this blog last September: http://durhammastergardeners.com/2012/09/05/dont-wait-to-renovate/
For even more information about caring for lawns, you can download Carolina Lawns here: http://www.turffiles.ncsu.edu/Guides.aspx#004175. The TurfFiles website is also an excellent source of information.
Homeowners with warm season lawns: Your lawns should NOT be seeded in fall. Sod can be laid in fall, but there is greater chance of winter kill if sodded at this time. Waiting until spring is recommended.