Monday

Monday follows Sunday because the planetary hour system that generates the week always advances the first hour's ruler three steps along the Chaldean order, the sequence of the seven classical planets arranged from slowest to fastest as seen from earth: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. Three steps on from the Sun lands on the Moon, so the Moon rules the first hour of the day after the Sun's day, and the day takes its name accordingly. Like Sunday, Monday kept its Latin planetary name directly into English, since mona, the Germanic word for the Moon, needed no substitution the way the days named for Mars, Mercury, Jupiter and Venus eventually received.

The Moon's Latin name, Luna, and its Germanic equivalent both describe the same visible body, which is why dies Lunae, lundi, lunedi and Monday all preserve a recognisably lunar name across languages descended from very different branches of Indo European, unlike the days whose Latin planet was replaced by an unrelated Germanic god's name.

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

Where Sunday carries the Sun's associations of vitality and the conscious self, Monday carries the Moon's: feeling, instinct, home, memory and the changeable rhythms of mood and need, following directly from the qualities classical astrology assigns to the Moon as the planet of the inner life. The Moon is also the fastest moving of the seven classical planets, completing its circuit against the fixed stars in about a month against the Sun's year and Saturn's three decades, and older astrological writing sometimes reads Monday's changeability, its reputation as an unsettled or difficult day, as an echo of the Moon's own restless motion.

In traditions that pair a working's success with matching its planetary day to a compatible planetary hour, Monday was considered the natural day for operations concerned with domestic life, emotional matters, dreams and anything tied to cycles and recurrence, mirroring the Moon's own governance of tides, growth cycles and the passage of time in monthly rather than yearly units.

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