For a healthy lawn, it is better to water every other day or every three days depending on your conditions (soil content, sun/shade ratio, rainfall amounts) and thoroughly saturate the soil. Watering less frequently and saturating the soil will encourage the roots to reach deeply for residual water, even when the surface of the soil appears dry.
The most effective way to water is before the heat of the day when the soil is cool and the water has the best chance of seeping down to the roots before evaporating. Watering early will ensure a sufficient store of moisture beneath the soil to withstand the heat of a hot summer day. This will also wash the dew off the grass, as dew beads up and creates a favorable environment for diseases to establish and thrive.
Rain isn’t always sufficient at supplying enough water for plants to thrive, so don’t count on it to keep plants healthy. A rain gauge is a great way to monitor the amount of weekly rainfall. If the gauge gets less than 1-inch of rain per week, supplement by watering. During the peak of summer, it is recommended to double that amount.