Proper water supply to your lawn can mean the difference between a lush oasis or a sandy desert.
When to water
If you lawn starts to take on a dull gray or brownish color it is telling you it needs water. Watering in the early morning hours (typically before 10 a.m.) is the best for your lawn. Early watering allows the water to soak into the soil and roots of the grass before it can evaporate. If for some reason you can water in the early morning hours an alternative is mid to late afternoon (typically 4 to 6 p.m.). This still gives time for any excess water to evaporate, you want the grass blades to be able to dry before night fall. Watering too much or leaving excess water on the grass can actually lead to grass related diseases which can spread very quickly and kill off you lawn.
How much
Most established lawns only need 1 to 1.5 inches of water a week which is usually best applied in multiple intervals once or twice a the week. The most important part is to make sure the water is penetrating the first 4 to 6 inches of top soil. As mentioned in Quick Mid-Summer Tips for Your Lawn the best way to measure the penetration depth is with the use of a trowel or screw driver.