Sanju123, the calendar is double Dutch to me too. I think the answer you get depends on whom you ask!
Here are a few different explanations being thrown around:
(1) The 7-day week stems from an astrological tradition of naming the days after 7 visible celestial objects: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. For example, in Hindi, Sunday is "Ravivār" and the literal translation is
"Sun day." In French, Thursday is "Jeudi" and the literal translation is
"Jupiter's Day."
(2) Some people say there are 7 days because the bible states that God created earth in 6 days, and rested on the 7th day. In Hebrew, Sunday is "Yom Rishon" which means
"first day." In Arabic, Monday is "yaum al-ithnayn" which means
"second day."
(3) There are other people who say that 7 days was chosen because that's the approximate length one moon phase.
Take your pick!
Or there are even more explanations
here and
here.