At what time of day should I water my lawn?

Early in the morning. If you water late in the evening the water will sit on your grass blades and can lead to fungus and other diseases. If you water in the middle of the day during the hot sun you lose alot of the water to evaporation and you risk scalding your grass. In the morning before the heat sets in allows the water to sink deep down into where the roots are and gives the blades time to dry.

Evening after the heat of the sun has gone down.

Morning is a good time to water. There are benefits to letting your soil dry very well on occasion between waterings. This helps to break down some of the minerals locked up in the soil.

I probably should relay some other interesting information which correlates.

Plants tend to "open" their system in the cooler parts of the day, especially in the morning. The "inhale" and "exhale", the exhange of gasses and nutrients often takes place then. This is a great time to foliar feed. There is no better approach to helping plants grow than an organic program...after all, it is organic just like you. Organic substances such as a compost tea (often composted manure based), liquid kelp (seaweed), some mollasses, mixed together and sprayed on plants at this time of day (or in the late evening if you can't make the morning) go an incredibly long way towards healthier and "happier" plants. Beneficial microbes (which feed and protect plants) are promoted with this mix. It strengthens the plant structure so it can better wwithstand times of stress, and inhibits pests and antagonistic insects or disease. Just like a human, the more naturally balanced and healthier the system, the better it can fend off disease. The mix is usually ounces in ratio 8-1-1 respectively for the compost tea (made by just putting the compost in a bucket and filling with water...strain/screen off your tea), molasses, liquified seaweed. From this mix, apply "one or two ounces" per gallon of water on your sprayer dial setting. ...but anyway...a point here is that plants are more receptive in their ability to intake nutrients during the cooler, less stressful part of the day. There is an additional factor: Plants being a life form, prefer aesthetic "atmospheres" or pleasing environments. The aesthetic morning sounds tend to "open" foliage. Birds and chirping and morning life sounds. Music (aesthetic type) does a similar type thing. Life gravitates towards the aesthetic, towards higher qualities such as affinity and appreciation. American Indians were know for their appreciation of nature and were in tune to it...and actually received some remarkable results. People who talk to their plants get good results. And, plants like to be of use...that is, to have a value such as for food or beauty or helping other life forms. I know...it sounds a little weird or "hippie esoteric". But the point here is that in the morning, there are often more aesthetic qualities which also help plants to open up.

By the way, synthetic fertilizers (and Miracle Grow is a major one) and other synthetic, unnatural chemicals impede the natural balance of plants. If a person tries to mix practices with synthetic and some organic, the synthetic approach often inhibits organic results. It is easy to discover "WHY" when you research what synthetics actually do to life forms.

depending on your area , the time of year and the amount of normal humidity in the air.
at night, early morning or late in the evening is the best times..

but some times like night may cause mildew etc on plants depending on your climatic area...check with a nurseryman in your area

Late after the sun goes down - That's how we do at the Golf Course

Early a.m. before the hot sun comes up.

Early morning.

If you have a timer and a sprinkler system, you should set you timer for 4:30 a.m. At this time in the early morning you will have the least amount of evaperation. Depending on where you live, you should set your timer from 15-20 minutes. You should also water every 2 to 3 days.

In the morning.

i've always been told either, early morning or late evening, Just as long as when they are wet the sun doesn't beat down on them and burn them up.

Evening I have found to be the best time that way the water has all night to soak into the ground before the sun comes up. I have found that by watering early in the morning when the sun comes up there seems to be alot more evaperation.

When the sun has just set.

I've been in the professional lawn care business for nearly 4 years now and have experienced alot. I work in chicago area where I manage cool-season grasses, but these grasses are exposed to extreme heat and insect stress in the summer, with heavy grubs in late summer/through fall. You can water either in the morning or at night, MOST IMPORTANTLY DEPENDING ON CURRENT CONDITIONS. Its all about HUMIDITY. If its humid, DO NOT WATER AT NIGHT. You are setting yourself up for a world of pain by letting fungus and diseses into your lawn which are much harder to deal with than any common turfgrass pest problem. If your humidity has been dry, water at night all you want. It is important to LET YOUR SOIL DRY OUT BETWEEN WATERINGS, before watering again to prevent these fungus/molds from coming in. Also keep in mind when your cutting your lawn below 3", you are lowering the amount of moisture your plant has uptaked from its root system by cutting your grass too short...You may be able to get away with it when temps dont exceed the 80 degree mark. But anything above 80....Keep the height high so your grass retiains the moisture, that way it can draw upon the stored moisture AND nutrients during times of stress. If you cut it below 3" plan on watering every other day for 20-40 minutes and fertilizing with quick release fertilizers every 4-6 weeks in the growing seasons, ESPECIALLY with sodded lawns. One of the biggest myths I've ever heard of is not fertilizing your lawn in the summer. This only holds true if you refuse to water your lawn(if you fall into this category, use a SLOW-RELEASE GRANUALE, the granuales will sit at the turf layer and activate when a T-storm comes in CHECK YOUR FORECAST FOR SUMMER RAINS!), however, if you give it 1-2 " of water each week, your lawn will GREATLY appreciate a nutrient feeding; during the summer, use half the amount of fertilizer you would use in the growing seasons, obviously you want nitrogen, but look for a fertilizer with a low amount of potash as well(the last number, i.e. (25-0-3) Try to stay away from using phosphrous fertilizers. If you mulch your lawn, the clippings should provide enough phosphorous naturually. Just make sure to NEVER leave clumps. Mulching your lawn and leaving clumps will end up damaging your grass, ever left an object on your lawn for a few days to pick it up and see the grass destroyed??? you see what I'm getting at...Last but not lease, CORE AERATE EVERY GROWING SEASON IF YOU CAN(On an already established lawn). The soil cores that are taken out (look like a poodle pooped all over the lawn) will break down when rain hits them, turning into a topdress which will naturally, organically break down thatch, and the left over clippings. Aerating also helps water and nutrients work about 15-25% more effectively by getting the water/nutrients directly to the root. Seed after aeration to get the seeds to drop in to the holes, this is great for people who live on slopes. PICK UP ALL YOUR LEAVES/DEBRIS BEFORE WINTER ARRIVES.