Grass seeds need a few vital things in order to germinate. One of those things is moisture. The average person probably prefers to seed their grass when it is dry outside for their personal convenience. They must then use supplemental water to provide the necessary moisture for grass seed to germinate. By planting grass seed in the rain you can get the seed to germinate the way nature intended--by allowing moisture to be applied naturally.
Watch the weather forecast and plan to seed your grass when a rainy period is forecasted. A steady, long-duration light rain is ideal.
Loosen the soil in the area you want to plant your grass seed with your garden rake.
Sprinkle the area with 16 to 22 grass seeds per square inch. Use a broadcast spreader if seeding large areas. Set it to the setting indicated on the grass seed bag to achieve the correct dispensing rate.
Rake the soil to cover the seeds. Burying them in the soil will allow them to achieve the necessary seed to soil contact required for germination and also prevent birds from eating them.
Watch the weather closely. If natural rainfall subsides, you will need to supplement extra water to keep the soil moist.