It depends on where you live and what kind of grass, there are warm season and cool season grasses. But generally the best time is early fall,( mid August to mid Sept.) next best is early spring as soon after last freeze as possible.. By planting in the fall the grass has a chance to set deep roots over the winter before summer heat bakes it. After planting you water every morning, keep soil moist,best is slow and deep, till about an inch high, then about every other day for a few weeks then as needed. That constant watering leads to shallow roots and vigorous growth and the water is in the top level of the soil but over winter the roots grow deeper. In the spring you water the same way, but when summer comes the heat dries up that top layer of water so you water more but it doesn't really sink down, the grass is growing vigorous and sucks up all the water it can get. You need to water slow like with a soaker hose so water goes deep. Use a slow acting root stimulating fertilizer when seeding, in fact work it into the soil before seeding, use a fertilizer with a high middle number, like 5-10-15 that's phosphorous and stimulates the roots. The first number is nitrogen and stimulates leaf growth, you don't want that till the roots are deep. If spring planting don't fertilize again till early fall then the high phosphate again. Then next spring and , if you fall planted, use a 10-10-10 or even a 10-5-10 or so. After that it's hi middle number in fall more balanced in spring and a quicker acting hi first during the summer. Uh, we are talking about LAWN grass, right? Otherwise the instructions are different, rule one is don't plant it in the front yard.