The best time of day to water your lawn is in the early morning. Moisture is able to reach the roots and yet the leaf surfaces are able to become dry. Watering at night or late in the evening creates a very humid environment and should be avoided. The excessive moisture, combined with high humid summer night temperatures, are conditions optimal for severe disease problems.
Watering during the heat of the day is not the most efficient time for the plant to be watered. Although it can cool the plant, it is not recommended because much of the water would be lost through evaporation before it has had a chance to be taken up by the grass plants. On established lawns, deep, infrequent watering is important in encouraging deeper rooting of the grass plants. Watering at least 1"/week, rather than light infrequent watering, will allow a good soaking of the turf area.
Newly seeded lawns need light, infrequent waterings to ensure good germination and establishment. Once the grass has germinated and has produced at least one leaf, waterings can be gradually cut back in frequency and increased in duration until the 1" requirement is reached.