The best time of day to water your plants can vary according to the climate you live in. In hot, arid climates such as those found in the Southwestern part of the United States, it's usually best to water your plants and gardens in the late evening or night time so the water doesn't evaporate before the soil is able to absorb it.
In Northern and Eastern parts of the United States, however, watering in the late evening can increase mold and mildew growth because the plants are too moist for too long. Watering in the morning is the best approach for those areas.
Try to avoid watering in the hottest and sunniest part of the day because your plants can burn. The sunniest hours are those that straddle noon: about 11am to 2pm in the Northeast and 10am to 3pm in the Southwest.
Apply water directly to the soil around the plant, being careful not to get any one the stems and leaves. Watering the soil will get water to the roots where it's needed, and keeping water off the stems and leaves will keep the plants from burning if the sun is too bright.
When water drops settle on the leaves in direct sunlight, they act as tiny magnifying glasses that enhance the strength of the sun and that's why the leaves can burn so easily in midday.
Consider installing a drip hose or soaker hose system so that you can always water your plants directly at the soil. You'll have the added benefit of reduced water waste and reduced water bills as well, because micro-irrigation systems don't allow as much of the water to evaporate into the air as sprinkler systems do.