The ideal time to plant grass seed is in the fall, preferably in late August or early September. This will give the seed plenty of time to take hold before the cold weather begins, which is important so that the seeds don't get washed away by rough snowstorms. At the same time, it is late enough that the hottest part of the summer is already over, so the blazing sun will not scorch the grass seed before it is able to take root. Keep in mind, however, that the grass will probably not thrive until the next spring.
Your second best choice is planting grass seed in the spring. Make sure to plant as early in the springtime as possible, right after the last snow of the winter has melted or in late March to early April. Waiting too long to plant will leave the tender plants vulnerable to the hot summer sun too early on. The new plants will also need endless amounts of water in the summertime if they are not developed enough.
Planting in the winter is the least preferable of the three options, but it may work if necessary. You'll need to plant at the very beginning of the winter, especially if you live in a place with plenty of snow. The seeds should stay dormant the entire winter and begin growing in the spring. Keep in mind, however, that if you choose this option, the first snow of winter may freeze the seeds before they have time to set down roots, leaving them open and vulnerable to birds and other seed-eaters once the snow melts. They also may wash away together with the melting snow.
Planting grass seed in the summertime is not recommended if you'd like to have the seeds grow successfully. Although it is possible, summer planting means that the seeds are open to very high temperatures and that any water added to the seeds evaporates extremely quickly. If you choose this option, it is essential to use electronic sprinklers or another method to keep the grass seed moist at all times.