The term light and darkness refers to the waxing and waning forces of Nature in their relationship to each other. These forces of light and darkness are balanced in the time of equinox. But, the forces of darkness are dominant at the Winter Solstice , the shortest day of the year; which, is why the new sun god has to be born in order to insure the return of light, the balance that is achieved at the Spring Equinox.
To insure the dominancy of the forces of light occurs at the Summer Solstice the days grow longer, after which the days shorten in order to surrender dominance to the forces of darkness. The shortening of the light of each day again establishes the balance between light and darkness, which insures the continuance of the Natural growth and decline cycle .
The forces of light and darkness do not exist just in Nature, but also are presence in the gods, and human beings. In some Witchcraft traditions a blessing of "May the powers of light and darkness be with you," which has a twofold meaning: It is meant to bestow balance to the individual and to align him/her with the cycle of Nature in a spiritual sense. Thus the person becomes centered in Nature and has a rapport with the deity. This person is now able to make choices that are not swayed by external influences.