Water twice a day when the temperature is above 80 degrees. If you have underground sprinklers, set the rotary heads for 20 minutes a zone and the misters for 8 minutes at 5:00 AM and 1:00 PM. I have heard the argument to not water in the heat of the day because you lose it to evaporation. Remember, your lawn is living and breathing. You cool down with a glass of water. We make sure our pets have plenty to drink. Why would you not want to cool down the grass?
Avoid watering late in the evening. You want to give your lawn time to dry before the sun goes down. If the lawn stays wet all night you are only inviting fungus and disease. Now, before I get flooded with emails from people without a sprinkler system, I should state that watering in the evening is still better than not watering at all.
Avoid too much water. I have also heard the argument to water deep and infrequently to make the roots go deeper. This argument is not based on science. The biological make up of cool season grass is that the roots grow deep when temperatures are 55 degrees to 70 degrees. That is whey the grass grows so fast in the spring, and also looks great in the fall. The roots are going to be more shallow as temperatures climb above 75 degrees, so don’t waste water trying to fight nature. A glass of water that is full cannot hold any more water, despite how long it is left under the faucet. Soil that is too wet will lead to diseases of the grass.
If you have an irrigation system, make sure to watch the heads to make sure everything is firing on all cylinders. When conditions get dry, gaps in coverage become obvious and green and brown circles indicative of poor irrigation coverage become obvious.