So there is no clear answer for this question but the Babylonians use a 7 days week theory which is made on the basis of the 7 major big stars or we can say the planets in the solar system. Those are Sun, moon, Mars, Mercury, Jupiter, Venus and Saturn. Each day in a week represent the each star. That’s why on the basis of this theory it says that 7 major big stars and planets comprises the seven days of the week. But as it is not clear, so many different theories are there in many different regions. So we cannot really say because it depends on the religious belief of the people or the historical documents that they have got on their historical places because each country has their own different history and theories that they believe in.