<s>[INST] How many days in a week ? [/INST] Under the Gregorian calendar system, which most of the world uses, there are seven days in a week.</s>\ <s>[INST] Why is it 7 days, has it always been 7 or did it change over time ? [/INST] It is difficult to pinpoint the primary source of this information, but some say the that around 2300 BCE, they could see 7 celestial objects with the naked eye (the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn). The Babylonians, known for observing the heavens, documented this, during the time of King Sargon I of Akkad. Other sources also say the Babylonians had instead documented the number of days it takes to complete each phase of the moon.</s>