Never water your lawn or landscape in the heat of the day. This may actually cause more harm than good. The droplets of water that remain on the grass, plants and flowers actually wilt the plant once the hot sun warms the water. Most of the water that is being put down is also being evaporated due to the heat.
It is best to water in the early morning or early evening hours, but never late in the evening. This will give the ground a chance to soak the water in and reach the root system of the plants. Watering late in the evening could possibly cause more disease and weed problems as they tend to develop more at night than during the day.