In general, you should water your lawn early in the morning. Watering early in the morning allows the grass and soil surface to dry off throughout the day. 

When you water your lawn during the middle of the day, especially during peak sunshine hours, a lot of the water will be wasted through evaporation.

Watering your lawn in the late evening or late at night is the worst thing you can do. When dew has already settled this creates the perfect environment for lawn fungal diseases to develop.