That depends on whether it is the entire lawn or just a bare spot. The best time to over-seed is early spring and early fall.
For bare spots, make sure to remove any old dead growth that still exists. Then scratch the soil surface at least 1-2 inches deep and level off. Apply your seed and cover with 1/4 inch of top dressing. Make sure to keep the area moist at all times until the seed has germinated and rooted. You should be able to mow the spot (on a high level setting) three weeks after the seed germinates and is visible.
Before over-seeding an entire lawn, mow your lawn to half its normal height. We recommend dethatching your lawn and removing all thatch debris before applying new seed. Then over-seed at the recommended over-seeding rate on the package of your lawn seed.
Cover the entire lawn with a 1/4 inch layer of the same top dressing mentioned above. Switch your watering cycle to at least twice daily (short cycles) the first two weeks, once per day the second week, every second day the third week and then back to your normal watering cycle from then on.
Try not to mow for at least two weeks after applying seed. Then do so at a higher than normal level setting until the new grass is well established.