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