In many regions, fall is an ideal time to fertilize and repair the lawn.
In northern areas, late-fall fertilizing (after the first frost) gives most cool-season turf grasses a head start the following spring. Use a slow-release fertilizer with an N-P-K ratio of 3-1-2.
Take the opportunity to aerate heavily trafficked and worn-out areas of the lawn. If the ground has been compacted, aerate and re-seed it well before the first frost so next year's grass will have a chance to root with less competition from weeds.
Use a power aerator or star-wheeled cultivator to till the target area to a depth of about 1/4 inch. Then, lightly coat the area with fresh soil, and sprinkle grass seeds to match the surrounding grass.
Sow about 15 seeds per square inch (check seed-box label for application rate). Till once again, and tamp lightly with your foot to get good seed-to-soil contact.
Use bamboo stakes or bent twigs to mark newly seeded areas, and water well.