Lawn watering during summer
Montgomery County Maryland
During longer dry spells and droughts, lawn watering mainly serves to keep the turf roots alive (assuming you have Fescue and not Zoysia). Cool-season grasses like Fescue do not actively grow in hot weather and naturally go dormant. This means that while the grass blades may brown some as a result, this is a natural response to a stressful season and should be accepted. Forcing them into active growth or a lusher green state by irrigation may only serve to stress them further and will promote disease outbreaks.
To irrigate to preserve root health (and incidentally water any nearby trees whose roots are in the area), about an inch of "rain" a week is typical advice. This means that, with a sprinkler, run it until a shallow can set in the area collects an inch of water. Depending on water pressure, this soaking could take an hour or more. Long, spaced-out soakings are much better than shorter, more-frequent soakings. (So for the "inch of rain a week" guideline, one good soaking a week will suffice. Because we need to tolerate dormancy, however, a soaking should last longer and be applied less often.) Morning is best because it hydrates the plants during the day (when they're using and losing the most water) and minimizes evaporation from afternoon watering. Evening leaf wetness promotes disease, so earlier watering allows the surface time to dry by nightfall.
You can feel the soil moisture by hand before determining if the yard needs water. Use a trowel, stake, sharpened wood pencil, Phillips-head screwdriver...anything that can get you around 4-6" beneath the surface. Moist soil will feel cooler, look darker, and stick to the skin (or the crevices in the screwdriver head) or moisten the wood on your probe. Drier soil will feel warmer (or not cool, at least), look closer to the surface soil in color, and brush off fairly easily.
If your sprinkler system is programmable, then it's best to leave it on manual (not on a schedule) for the sake of adjustments due to rain, changes in soil drying, etc. Some manufacturers build-in rain detectors so they automatically delay the schedule if enough rain falls.