Is your lawn a warm season lawn or a cool season lawn?
If it's a warm season lawn, I believe the best time of year to establish (or repair) is mid spring to very early summer. For a cool season lawn, the best time of year to establish (or repair) is from August 15th to September 15th (exact date will vary slightly depending on the growing zone you're in).
Niall C.'s answer is a good one, but if you require more detailed information regarding the establishment of a cool season lawn, you may wish to take a look at the following University of Missouri Extension document - Cool-Season Grasses: Lawn Establishment and Renovation.
Some info that is relevant to your question:
Newly seeded or sodded lawns require special irrigation. A newly seeded lawn requires daily watering and may need as many as four light waterings in a single day if conditions are dry and windy. Keep the seedbed moist, but not saturated, to a depth of 1 to 2 inches until germination occurs (green cast to lawn and seedlings 1/4 to 1/2 inch tall). At this stage, it is crucial that seedlings not be stressed to the point of wilt.
Also since you're repairing your lawn, this part might be relevant to you:
Prepare surface for seeding
One of the most important steps in renovation is placing the seed in contact with soil. This sounds simple, but most lawns have thatch - an intermingled layer of both dead and living plants - over the soil surface. This brown, decomposing layer may be up to 1 1/2 inches thick. Lawns with more than 1/2 inch of thatch should be dethatched.
Seed placed on or in the thatch layer may germinate, but the eventual stand of grass that develops will be poor. To ensure good seed-to-soil contact, use power equipment to prepare the surface for seeding. Power rakes, verticutters, slit seeders and core aerators are effective machines for properly preparing the surface for seeding.