And it was said: To indicate the transcendence of the manifestation represented by it from both darkness and light, for each of them causes the other to vanish—as the Exalted pointed to this meaning in Surah Al-Nur by His saying: {Allah alternates the night and the day. Indeed, in that is a lesson for those of vision} (Al-Nur 44).

The Exalted made it night and day for many meanings, among them:

And it was said: It is to indicate the obligation of revealing the knowledge of the manifestation—represented by the Day—to the believers, and the obligation of concealing it—represented by the Night—from the disbelievers. Apply this measure to all other days and nights, and all manifestations represented by them and their occultations, measure for measure.

As for applying the name "Day" to Sunday and other days, it is like applying the name "Manifestation" to all the stations of the Name and other stations of the spiritual ranks of the Meaning, the Name, and the Gate. The same applies to the names of the months, for each of them is called a "Month," just as each of the days is called a "Day." The indication in this is to the essence of the Person represented by it and to his manifestation represented by the Day.