When over-seeding our lawns here in the great Pacific Northwest we need to take a few things into consideration for planting to be successful. The first one is timing and its most important because of temperature. In order for seed germination the day time temperature needs to be between 55 - 75 degrees Fahrenheit consistently making late spring and fall the optimal time. The type of grasses found here are cool season grasses that are only active during spring and fall and dormant in the summer and winter. I'll explain more why this important a little later below.
Second, a good starter fertilizer should be used when spreading the grass seed. I use a 18(nitrogen) - 24 (phosphorus) - 6 (potassium) blend.. I like to use a higher phosphorus content to help promote a stronger, deeper root system that I believe is important and will feed your lawn for up to 12 weeks.
The third one is moisture. It is very important to keep the seedlings damp but not soggy, if at any time during the watering cycle that water starts to sit on top of the soil it's too much. As the seedlings start to grow and you reach the 12-15 day mark cut back on watering but water more heavily when you do. Using this practice will help the roots grow deeper and become more established creating a greater tolerance to wear and the ability to fight off disease better.
This process is best done after core-aerating your lawn. This helps the seed and fertilizer get down in the lawn and make contact with the soil via the holes. Aeration should be done during the active growing seasons that are mentioned above so the grass can easily heal its self from the trauma of the process. By coupling aeration, over-seeding and fertilization together it makes spring and fall the best time to rejuvenate your lawn.