When does the law require the headlights on vehicles to be turned on?

A Between half an hour before sunset to half an hour after sunrise and at any other time you cannot see clearly for a distance of 150 meters (500 feet)

explanation

Turn on the headlights one-half hour before sunset and keep them on until one-half hour after sunrise. You must always use the headlights when fog, snow, or rain keeps you from clearly seeing people or vehicles less than 150 meters away.