As the Moon orbits the earth, its lit area appears to change constantly. The most identifiable phases are when the Moon is full and when exactly half of it appears lit. These account for three of the Moon's four "quarters". The fourth is when the Moon as viewed from the Earth is completely dark. The time period between these quarters is roughly seven days, which is the origin of our week.
The week-days are associated with the seven wandering lights: the Sun and Moon, and the five planets that are visible to the naked eye. The obvious associations in English are Satur(n)day, Sunday, and Mo(o)nday. The others (from associations with the old Norse pantheon) are Tuesday - Mars, Wednesday - Mercury, Thursday - Jupiter, and Friday - Venus. This arrangement lets the Cosmic Couple (the Sun and Moon) be together while preserving, though obscuring, the scientific arrangement of things. The wanderers were ordered by the scientists of yore according to how fast they appeared to move relative to the backdrop of fixed stars. This order is revealed by starting with Monday and skipping counterclockwise around the circle of week-days: Moon, Mercury, Venus, Sun,  Mars, Jupiter, Saturn (Mon.,Wed.,Fri.,Sun.,Tue.,Thur.,Sat.).
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