Morning is the best time to water most lawns. Before 10 am is best of all because rising heat later on tends to steal a lot of water by evaporation. There is another good thing about early watering; the grass leaves have a chance to dry off quickly. Evening or nighttime watering leaves the grass wet and under cool conditions, and that can increase the chance of your lawn developing a disease.
Most other plants benefit from early hour watering too. Scald or burn damage can result from hot sunlight hitting water droplets left on the leaves. They act just like little magnifying glasses. Early or later, try to give the lawn a good deep soaking when you water, not a light sprinkling. Light watering is actually least effective and can even be harmful to your plants. The water does not penetrate the soil deeply enough to encourage plant roots to grow firm and solid. Your lawn will be less drought-resistant in summer and more prone to winterkill later.
A healthy lawn only needs about 1-inch of water a week, even during the hot summer months.
You can easily measure how much water your using. Put some small empty containers (such as empty soup cans) around your lawn before you start watering. Time how long it takes to fill a container with 1 inch of water. That is how long you should water your lawn each week. It will also help you spot areas that are getting too much or too little water.
This 1-inch of water is a guideline, and you should adjust your weekly watering by the amount of rain your lawn receives.
If your lawn is on a slope, you should slow the flow-rate of the water so the lawn can absorb it and not just let it run off.
Good: Watering to one inch per week.
Better: Only watering when necessary, not exceeding one inch per week.
Best: Allow your lawn to go dormant and rely on rainwater. When cooler temperatures and increased rainfall return, your lawn will recover.