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 (planets) were known to the ancients: the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn. These celestial bodies are visible to the human eye; people in ancient times were able to observe how they moved across the sky. It is also where the term 'planet' comes from; it is originated from the Greek word meaning 'wanderer.'
The names that we use for the days of the week originated from different cultures and mostly referred to one of the seven celestial bodies or the planet-god watching on a particular day.
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 planet. In Italian, this day is called "Mercoledi," or Mercury in English. Donar, the Germanic god of thunder (Thor, in English), watches on Thursday. In this case, Donar represents the planet-god Jupiter.
The planet Venus appears in the word "Venerdi," 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; in English, the word '"Saturday" and the planet Saturn sound very much alike.