Seeding a new lawn is the most popular way of creating a new lawn simply because it is so inexpensive compared with other turfgrass methods, and it is relatively easy to do. Grass seed can be planted almost any time during the year with a few exceptions and caveats. Other than putting down seed at certain times, the main factor is what comes after the seed sprouts? The best time for planting grass is at the beginning of its growing cycle. For cool season grasses, there are 2 growing cycles: FALL and SPRING. For warm season grasses, there is 1 growing cycle: SUMMER.
For most cool season grasses and for folks living in northern climates, the best time to plant grass is in late August, early September. Depending on exactly the climate you have and the type of grass seed you're planting. Some grass seeds take longer to germinate and develop a strong enough root system to withstand the first cold snaps before going dormant for the winter.
Bluegrass has an exceptionally long germination period and slow development, which would be better if started in the spring. However, doing this means that you will have to take greater care in providing adequate moisture throughout that first growing season, especially in the hot summer months.
One very easy to do technique, especially if you are going to be overseeding a thin lawn in the northern climate where the ground freezes, is to spread seed in late winter, when the ground is frozen, but NOT snow covered. In late winter, the ground typically freezes and thaws with every night/day cycle. Grass seed will not germinate until the soil reaches about 55 degrees, so you don't have to worry about your grass starting to grow and then being frozen -- it won't happen.
After you spread the grass seed on the frozen ground, the ground will eventually thaw, and then refreeze. With each of those freeze/thaw cycles, the ground heaves up and settles back down. During this process the seed is gently pulled into the upper layer of soil where it will remain until the temperatures moderate later in the spring.
Warm season grasses grow best during the hot summer months. That makes spring and early summer as the best time to plant warm season grass seed. Following germination make sure the new plants have adequate water during the remainder of the growing season. Most warm season grasses are typically planted with live plants instead of seed. This gives the lawn a better chance of survival during the hot months. Also, many warm season grasses are hybrids that don't produce viable seeds. That means the only way to grow them is through transplant (sprigs and plugs) or through laying sod.
Besides planting the right grass seed at the right time, the next step is making sure the soil is adequately cultivated. For bare soil, the soil should be tilled at least several inches, but 4" - 6" is preferred. This will require using a tiller which can be a strenuous activity. Debris should be removed and the soil leveled using the back of a rake.
Before spreading seed, put down a special starter fertilizer according to directions on the label. High quality starter fertilizers usually contain slow release granules that let their contents to be leached out of their granules over a period of weeds.
Spread the seed according to the rate suggested on the seed package. Make sure it is EVENLY spread. Too much seed at any one spot will prevent the newly emerged seedlings from properly developing during the following months and they will usually die off. Using a drop spreader is probably the most accurate method of spread grass seed at a predetermined rate. Hand spreaders that throw seed out are more difficult to control the spread rate.
TIP: When using a spreader, cut the recommended rate setting in half, then go over the same area a second time in a different direction. This will help provide an even distribution.
After seeding, go over the soil with the backside of a rake -- LIGHTLY! You only want to cover the seed with about 1/4" of soil.
After spreading the seed and gently raking it in, you'll want to put some type of mulch over the soil to moderate soil temperatures and moisture loss. Straw (not hay) has always been a good mulch. When spreading the straw, you want to be able to see about 1" open spaces of ground. This isn't rocket science, but the point is not to completely cover the ground. Sunlight needs to reach the soil more or less. Think of it as creating a small trellis over your new lawn.
During the germination days, make sure the soil remains evenly moist, but never wet. You don't want the soil to ever dry out completely and you don't want to soak the lawn to a point where water is standing.
After the seeds germinate, you'll want to again keep the soil evening moist, but as the seedlings begin to develop, you can cut back on daily watering's, and go to every other day, with slightly deeper watering's. Continue this until the grass reaches about 2" - 3" at which time you can completely cut back on watering, just making sure that the lawn gets about 1" per week total.
Leave the straw in place, and it will eventually decompose. Don't worry about weeds that may sprout during this time. You can control them next year after the new lawn has fully developed.