Water so that the soil under the grass is damp 6 to 8 inches down. Apply water slowly and uniformly as to prevent run-off. If it becomes saturated and is only 2-3 inches down, wait about 30 minutes before resuming. Too frequent watering, with out proper depth, will promote shallow root growth on the lawn and make it susceptible to weeds and disease. When to water is a matter of preference.
Many people prefer morning, while others prefer the evening.
Unless your lawn is otherwise healthy, it is best to water in the morning. You can water in the afternoon if it is convenient, however, be aware that the high temperatures and wind tend to evaporate water quickly and it may end up costing you more in the long run.