If you have an automatic sprinkler system, the best time of day to water your lawn is very early in the morning, right about when the sun comes up.

It's amazing to me the number of sprinkler systems I see on during the middle of the day. If you water your lawn in the middle of the day, you lose a lot of water to evaporation as it is typically hottest at this time. Also in the middle of the day, the winds tend to be higher, so you will lose additional water to the wind.

If you water later in the evening, the evaporation and wind problems are reduced, but the water then sits on your lawn all night long. This prolonged moisture can lead to molds, fungi, and other nasties in your lawn.

If you water early in the morning, there tends to be light wind. Since it's still cool, you don't lose much water to evaporation. The water has time to soak in, and the oncoming heat will evaporate the water from the surface (where there are no grass roots anyway) which will keep molds in check.