When should the headlights be turned on?

Your headlights should be on whenever it will make a difference on the road. If you can look at the road and be able to tell that your headlights are on/off, then they should be on.

Some states, such as New York, require headlights to be on when visibility is limited, such as in rain or fog, even in the day, regardless of whether or not you can see them on the road. In fact, one of the criteria for headlights is whether or not you are using your windshield wipers.

At dusk or, in some states where mandated, when it is raining or foggy and you need to turn on your windshield wipers.