Grass seed needs moisture to germinate. What seems like overkill for any other plant is just about right for grass seed, as it must remain nestled in damp soil at all times through germination and the root establishment phase. This means continuous close observation of moisture conditions and daily watering. In warmer or drier climates, even more frequent watering may be needed.
Watering at Sowing Time
After the grass seed has been sown, the soil and seed should be thoroughly soaked with water so that the seed is nestled into the soil and the soil is damp at least two inches down. Use a hand-held hose attachment sprayer that has an adjustable nozzle. Use a mist or light gentle rain setting so you don't disturb or resettle the seed.
Establishment Phase & Maintenance Watering
You need to keep the top inch of soil constantly damp through the seed germination and root establishment phases. This will likely require you to water the entire seeded area lightly but thoroughly at least once or twice a day. Make sure you get the edges as well. During the heat of summer or in very windy climates, you may need to water three times a day.
Continue to keep the soil consistently damp until the blades of grass are two to three inches in height. You can then gradually scale back to watering every three to five days for a month or so to support the development of a root system. By the second month, less frequent, long, deep watering is better for healthy root function and a maintenance watering regimen of every seven to 10 days should be established.