Sunday

The seven day week did not begin as a religious institution but as an astrological one: the planetary hours, cycling the seven classical planets through the twenty four hours of each day in a fixed order, always land the first hour after sunrise three planets further along than the day before, and naming each day for the planet ruling its first hour is what produces the week itself. Sunday takes the Sun's hour first, so it is dies Solis, the Sun's day, in the Latin that spread this system across the Roman world. Most of the Germanic languages later replaced the Latin planets with roughly equivalent native gods, a process called interpretatio germanica, but the Sun and the Moon needed no substitute: sunne and mona were already the ordinary Germanic words for the bodies themselves, so English simply carried the Latin day name across intact rather than swapping in a god's name as it did for Tuesday through Friday.

Christian usage layered a second meaning on top without displacing the astrological one, reading Sunday as the Lord's day in honour of the resurrection, a naming convention that spread through Romance languages as domingo, domenica and dimanche while the Germanic languages, English among them, kept the older planetary name in everyday use. The result is a day whose calendar position is fixed by an ancient astrological reckoning and whose religious significance was added centuries later onto a name that already existed for other reasons entirely.

Sources Norse Mythology: A Guide to the Gods, Heroes, Rituals, and BeliefsJohn Lindow with Deutsche MythologieJacob Grimm

Classical and medieval astrology reads each day of the week through the character of its ruling planet, and Sunday carries the Sun's own significations: vitality, the conscious self, authority, honour and new undertakings. A working timed to Sunday, in the older magical and astrological literature that pairs planetary days with planetary hours, was considered strengthened when it also fell within an hour ruled by the Sun, the doubled attribution reinforcing rather than merely repeating the day's own character.

Because Sunday opens the seven day cycle, it also carries associations of beginning that have less to do with the Sun's specific nature and more to do with its position: the first day of a new week inherits some of the symbolism of any first thing, a fresh start set against the six days that follow and complete the cycle before it starts again. That double association, solar vitality on one hand and simple positional primacy on the other, is why Sunday appears in astrological and folk tradition alike as an auspicious day for beginnings even in traditions that do not otherwise emphasize planetary rulership.

Sources A History of Western AstrologyNicholas Campion with Grimoires: A History of Magic BooksOwen Davies