It’s in the morning before 10 am that soil absorbs water most efficiently. That’s because both the air and the soil are cooler and that reduces evaporation. Therefore your plants get the greatest benefit from the water you apply. In addition, the soil’s surface will have all day to dry out, much to the chagrin of slugs, who prefer soil that is moist at all times. In fact, studies have shown that watering in the morning can reduce the slug population by up to 80%!
The second best time to water is in the late afternoon or evening or just after sunset, because at least the sun is no longer beating down on the soil and the plants, reducing evaporation somewhat. Unfortunately, the air and the soil are still warm, so a lot of evaporation is still going on. Furthermore, late waterings tend to leave foliage moist all night and that can lead to plant diseases (moist foliage is a prerequisite for the development of most fungal and bacterial diseases). Try to water without moistening the leaves. There isn’t much you can do, though, to dry the soil out. If you water late, it will remain moist all night, exactly the condition slugs adore!
The worst time to water is, logically, in the middle of the day. When the sun is hottest, evaporation will be at its peak and even less water will get to your plants than in the afternoon (up to 70% of the moisture applied can be lost under certain circumstances). Obviously, if your plants really need watering and you’re only available in the middle of the day, go right ahead, but you’ll need top water longer and use more water to really meet their needs.
Definitely, the morning is really the best time to water!