History of the week

The 7-day week began in the Middle East. Babylonia kept track of the phases of the moon. Lunar calendar months begin with the first crescent. From there to the first half moon was 7 days, then 7 more days until the full moon, another 7 until the last half moon, then 7 more until the last crescent, followed by 1 or 2 intercalary days of no moon, which might have been added to the last week. This was apparently adopted by the Jews while in exile, as the Hebrew word for the 7th day, Sabbath, comes from Sabattu, meaning full moon, or literally, "middle rest" day in Babylonian.

The average month is 29.5 days, with no moon visible the last day or two, called "intercalary," so two consecutive months would look like this:

The Jews stopped counting the extra days, ignoring the moon, making each week exactly 7 days although they had a mostly lunar calendar. They don't have names for the other days, but simply number them, 1–6. Their mythology explains the week's origin as the time it took God to create the world. 

The week spread to other cultures. The Greeks based the days of the week on astrology, naming each day for a different celestial body, or planet, each named for different Greek god. 

The Romans derived names from the equivalent Roman gods: Sol, Luna, Mars, Mercury, Venus, Jove, and Saturn, and modern Romance languages use names derived from these. The Romans originally had a lunar calendar, with the first day of the crescent moon called "kalends," the first quarter "nones," the full moon "ides," and then about 15 days until the next month. These names continued with the Julian solar calendar, although it ignored the moon. They also used an 8-day market week, which was gradually replaced by the 7 days we're familiar with. 

In English, we still use the celestial names for Sunday, Monday, and Saturday. The other days are named for the Germanic equivalents of the gods: Tiw's Day, Woden's Day, Thor's Day, and Frigg's Day.  

During the French Revolution, they created the decimal metric system, and replaced the 7-day week with 10 days, until this was abolished by Napoleon. The Soviets briefly introduced a week of 5 days. 

The 7-day week is now universal, used in every country, regardless of religion or calendar.