The correlation between the seven celestial bodies and the seven days of the week
Even though it would be perfectly possible to imagine a week having five, six or even eleven days, most cultures in the world have seven-day weeks. The reason for this is that seven celestial bodies were known to the ancients: the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn. These celestial bodies are visible to the human eye and people in ancient times were able to observe how they moved across the sky. This is also where the term 'planet' comes from - it is derived from a Greek word meaning 'wanderer'.
The names that we use for the days of the week originated from different cultures and mostly refer to one of the seven celestial bodies or to the planet-god watching over a particular day.
The names for the days of the week are derived from celestial bodies
Sunday and Monday are quite simply the day of the Sun and the day of the Moon. Only the English name for Tuesday still provides us with a clue as to the planet this day is named after: Mars, the Roman God of War, was known as Tiu to the Germanic peoples. 'Day of Tiu' or 'Tiu's day' eventually became 'Tuesday'.
The next day of the week, Wednesday, also has a connection with the planets. In Italian, this day is called 'mercoledì', a word in which the name of the planet Mercury can be recognised. Donar, the Germanic god of thunder (Thor, in English), watches over Thursday. In this case, Donar represents the planet-god Jupiter.
The planet Venus appears in the word ‘venerdì’, the Italian name for Friday. In English and German ('Freitag'), Friday was named after the Germanic goddess Freya. The German name for Saturday ('Samstag' or 'Sonnabend', the latter meaning 'the day before Sunday') derives from the Sabbath, the Jewish day of rest; only in the English word 'Saturday' can the name of the planet Saturn still be recognised.