A good time to water your lawn is between the hours of 6 and 8 a.m., according to SFGate.com. Watering during these hours reduces the possibility of fungal problems and cuts down on evaporation.
The worst time to water a lawn is during midday, because the temperatures are the hottest, which causes much of the water to be evaporated before it soaks into the grass. The evening is also a poor time to water, because the lawn stays wet through much of the night, which can lead to fungal diseases. It's best to also avoid watering during windy weather, because the water evaporates and is distributed unevenly.