When watering your lawn it's best to water the lawn early in the morning before the sun comes up and before it gets too hot.
If you water the lawn late in the day or during the noon hours then the water could evaporate before it reaches the grasses roots.
So water early in the morning or if you cannot water the grass early in the morning you can water the grass late in the evening.
I water my grass early in the morning and sometimes later in the evening after I get home from work especially when the weather is hot.
You don't want to water the grass when it's hot unless you're using a sprinkler and trying to use the sprinkler to just cool down with then that's fine.
But if you're just watering the grass then it's best to do it when it's not so hot out.