Evening, after the sun is off the plants. Some people do it very early morning, but then when the weather is hot, the sun comes up and catches the wet plants/lawn and that's not very useful. Doing it in the evening means its got all night to soak in before the sun rises again.
Watering of hardy plants in the ground, including lawns, should not be carried out daily though - if you want more info on that, click on my avatar, select blogs on my profile page and then select Watering for Beginners - not that I'm suggesting you're a beginner as such, but it does clarify how much and when watering should be carried out.

I water in the evening to make my watering count as in the hot weather evaporation is great. But not very often once the plant is established, to encourage the roots to go down and look for water. I do not water my lawn as it always recovers no matter how dry it gets and I can live without a green lawn in hot dry weather which I think never looks right in high summer anyway.